I am trying to supplement Ken Graham's excellent answer because zippy2006 says it has not addressed the core issue posed in the question. My purpose is more of giving perspective, not trying to be canon lawyer, theologian, liturgist, or procedural expert.
"If an obligation that would ordinarily bind under pain of mortal sin can be dispensed with, then is it really an obligation that ought to bind under pain of mortal sin?" I don't think it takes much effort at all to understand what the title means. Indeed, many are fixated on the technical sense of "dispense" without taking into account the organic meaning of the word. The title expresses the essence of the question and gets at the central issue of whether what is being prohibited is gravely sinful. Whether dispensation can apply to sin-prohibitions is precisely the point of the question.
I think we need to consider why the obligation exists in the first place, before talking about dispensation. Why does the Catholic Church consider it a grave matter to attend weekly Sunday Mass in the first place? (Note: see also my answer to a follow up question on more of the history of the obligation.)
The March 1997 article "Attendance at Mass and participation in the liturgy" from The Angelus magazine explains in a great detail the essence of attending mass as well as helping us to understand the essence by attending to various circumstances, some of which give light to why the Catholic Church currently dispenses her members from attending Sunday Mass due to Corona Virus. Some quotes from the article will follow the answer.
The purpose of attending weekly Sunday Mass is "to prove to God that [the faithful] really loves Him and wants to receive all the graces necessary for his eternal salvation." To miss these graces is literally a matter of spiritual life and death. That is why the Catholic Church consider it grave matter, and appropriately assigned "mortal sin" for violating it. But can the Church force us to love God? This is between us and God. So what can the Church do?
I think the Jewish practice of coming up with the extra prohibition/obligation as a "fence" to protect Jews from violating the 613 commandments can be used here as an analogy. We humans have to battle laziness, distraction, concupiscence, etc. and in Her wisdom the Catholic Church knew that we would rather stay home and do other things than going to mass. So in aiming at leading all faithful to Christ, the Church decided to promulgate this obligation as a "fence" herding unruly sheep to the right path so we meet God properly every Sunday so we have better chance to be in the state of grace.
By dispensing the Sunday obligation, the Church is in no way diminishing the seriousness of the purpose of the Sunday Mass in the first place, which is "to prove to God that [the faithful] really loves Him and wants to receive all the graces necessary for his eternal salvation." It is just that the mode of herding has to be temporarily adjusted in light of the circumstances. Just like murder, not loving God is still a mortal sin. Nothing changes here. Now that the faithful cannot go Sunday mass, the faithful still has the obligation to show God our love and still has to be replaced in a similar manner as people who qualify for one of the 3 exemptions that have been in place before the Corona virus dispensation.
Therefore, as Ken Graham has explained through another reasoning, the essence of the grave matter causing the mortal sin is unchanged, making the example of Corona virus irrelevant for your question.
Quotes from the article (emphasis mine)
The Church aims at leading all men to Christ, Who gives all honor and glory to God the Father in union with the Holy Ghost. This is, in fact, the reason for our existence: to glorify God, i.e., by recognizing His transcendence, His majesty, His power, His goodness and by singing His praises.
And no human being can do this except through Christ, with Him and in Him. This is likewise the first purpose of the Holy Sacrifice of the Mass.
That is why everyone should go to Mass: not just because they must or to avoid committing a mortal sin, but in order to participate in the great liturgy of our Mother the Church, Who wants to gather Her children for this great “action” of Her Spouse. This “action” proclaims to God the Father that His children love Him as well as His beloved Son.
If we would better understand why there is a Church precept to go to Mass, let us recall to mind here the fourfold purpose of the sacrifice of the Cross, which is the same purpose as in the Holy Sacrifice of the Mass.
- The first is adoration: to acknowledge the greatness, majesty and power of God, and to humble oneself before Him.
- The second is thanksgiving: to thank Him for the many gifts, both universal and personal, for many graces, for so much goodness and mercy towards us, poor sinners.
- The third is propitiation, to make reparation and expiation for our faults, and to implore pardon of God for all sins, whether known or unknown, whether committed by ourselves or by others.
- The fourth is petition: to expose to God all our needs and to pray to Him for those who need His graces.
No human being can afford to neglect the accomplishment of these four duties, if he wants to prove to God that he really loves Him and wants to receive all the graces necessary for his eternal salvation. However nobody except our Lord Jesus Christ can wholly fulfill these duties, Whose homage is pleasing and acceptable to God, because of the purity of His love and the offering of Himself. Our Lord Jesus Christ comes to us with the holy and pure gift of Himself, through the hands of the Church and her ministers. He invites us to present our gifts, miserable and imperfect though they be, to combine them with His own so that they may be acceptable to God the Father.
Thus, Sunday Mass summarizes all our prayers from the previous week, and all our efforts and meritorious acts, even those of the following week are contained in the holy oblation of our Lord in such a way that they are rendered pleasing and acceptable to His Father. Isn’t this an outstanding proof of goodness and tenderness? Do we really need to be forced by the Church to go and have recourse to our Lord, to tell Him that we love Him, to unite ourselves to His sacrifice in order to receive His Divinity in return, and His soul, His Body, His Blood, His life given up for us, His patience and power, strength and goodness together with the immense blessing which the Father has reserved for his Son and for all those who resemble Him?
Our Sunday Obligation
Let us remark first of all that this precept is not restrictive. True, it is both allowed and even encouraged to spend an important part of Sunday in religious activities in order to better sanctify Sunday, e.g., to assist at a longer Mass rather than a short one, to attend Vespers or Compline; to spend more time with the family in relaxation, music, walks, games, prayers, good readings or visits to a shrine....
But the Church obliges, strictly speaking, only attendance at Mass. Moreover, this is an obligation for each and every Catholic aged seven and older, under pain of mortal sin, and there are certain conditions to be fulfilled which follow. We will not speak here about dress code, although this is likewise an important issue.
Although the Church cannot exempt us from keeping Sundays holy, since this is a divine precept, She can dispense us from doing it by assisting at Mass, an ecclesiastical precept.
There are three causes which may exempt from the Sunday obligation:
Necessity: physical or moral impossibility such as sickness, convalescence, nervous problems because of the crowd, too far to drive, etc.
Duties of state: soldiers, doctors, nurses, firemen on duty, mothers with very young children, etc.
Charity: the needs of our neighbor, such as taking care of the sick, etc.
In all these cases it is not required to make up by attending Mass during the week, even though that would be laudable; but it is still necessary to sanctify Sunday in some other way as best one can. The best thing would be to spend as much time privately in prayer, as one would have otherwise spent at Mass on Sunday.
The above remarks apply only to Masses which are an obligation. There is no sin whatever in missing parts of a weekday Mass.