Have any theologians or denominations argued against the 5-day work week on scriptural grounds?
To my knowledge, no theologian or denomination has argued against the 5-day work week on scriptural grounds!
It is nowadays commonplace to work a 5-day week or 37.5-40 hours a week.
The Scriptures themselves call for a sabbath rest every seven days! But it equally does not say we can have two days away from work as a rest either.
Six days you shall labor, but on the seventh day you shall rest; even during the plowing season and harvest you must rest. - Exodus 34:21
Even Our Lord Jesus Christ Jesus said, “The Sabbath was made for man, not man for the Sabbath.”
And he said unto them, The sabbath was made for man, and not man for the sabbath. - Mark 2:27
What does it mean that the Sabbath was made for man and not man for the Sabbath?
Scriptures tell us to keep Holy the Sabbath, , but it equally doe not forbid us to take an extra day off.
To show that the concept of having a extra time away from work is not a new idea, one simply has to look up the history of what was an octave in the times of days gone by.
In some ancient octaves, work was not done.
Celebration of octaves in ancient and modern times
The liturgy of the octave assumed its present form slowly. In the first period, that is from the fourth to the sixth and even seventh century, little thought seems to have been given to varying the liturgical formulæ during the eight days. The sacramentaries of Gelasius and St. Gregory make no mention of the intervening days; on the octave day the office of the feast is repeated. The dies octava is indeed made more prominent by the liturgy. The Sunday following Easter (i.e. Sunday in albis) and the octave day of Christmas (now the Circumcision) are treated very early as feast days by the liturgy. Certain octaves were considered as privileged days, on which work was forbidden. The courts and theatres were closed ("Cod. Theod.", XV, tit. v de spect. leg. 5; IX, de quæst. leg. 7; "Conc. Mog", 813, c. xxxvi). After Easter, Pentecost, and Christmas had received octaves, the tendency was to have an octave for all the solemn feasts. Etheria speaks of the feast of the Dedication (cf. Cabrol, op. cit., pp. 128-9). Theodomar, a contemporary of Charlemagne, speaks only of the octaves of Christmas and the Epiphany but it must not be concluded that he was ignorant of those of Easter and Pentecost, which were more celebrated.
The practice of having octaves for the feasts of the saints does not seem to be older than the eighth century, and even then it was peculiar to the Latins. From the ninth century it becomes more frequent. The capitularies of Charlemagne speak of the octaves of Christmas, the Epiphany, and Easter. Amalarius, after mentioning the four octaves of Christmas, the Epiphany, Easter, and Pentecost, tells us that it was customary in his time to celebrate the octaves of the feasts of Sts. Peter and Paul and other saints, "quorum festivitas apud nos clarior habetur . . . . et quorum consuetudo diversarum ecclesiarum octavas celebrat" (De eccl. offic., IV, xxxvi). In the thirteenth century this custom extends to many other feasts, under the influence of the Franciscans, who then exerted a preponderating influence on the formation of the modern Breviary (Bäumer-Biron, "Hist. du Breviaire", II, 31, 71, 199). The Franciscan feasts of Sts. Francis, Clare, Anthony of Padua, Bernadine etc., had their octaves. At the time of the reformation of the Breviary (Breviary of St. Pius V, 1568) the question of regulating the octaves was considered. Two kinds of octaves were distinguished, those of feasts of our Lord, and those of saints and the dedication. In the first category are further distinguished principal feasts — those of Easter and Pentecost, which had specially privileged octaves, and those of Christmas, the Epiphany, and Corpus Christi, which were privileged (the Ascension octave was not privileged). Octaves, which exclude all or practically all occurring; and transferred feasts, are called privileged. The octaves of saints were treated almost like that of the Ascension. This classification entailed the application of a certain number of rubrics, the details of which can be found in Bäumer-Biron, op. cit., II, 199-200.
Today octaves are generally celebrated liturgically only. The days of eight (8) off from servile work at Easter or Pentecost are long gone.
Forgotten Customs of the Octave of Pentecost
Nevertheless, some bloggers do speak about a 4 or 5-work day week: