If some maniacal, evil, "hitler-esque" dictator decided that he wanted to cripple the Catholic church by assassinating all the bishops, and then he actually succeeds at killing them all, how would the church respond? With no Bishops to perform ordinations, It wouldn't take long for all the priests to die off completely. The church would still exist, however it would find itself in a position where it no longer has the power to perform the sacrifice of the mass, no longer has the ability to distribute the Eucharist to the faithful, no longer has the power to hear confessions. These are all very important aspects of Catholicism! What impact would these things have on the Church and how would it respond/survive/adapt to the new situation? I suspect it would start to look a lot like protestantism, as the only lords supper that would be possible would be a symbolic one and the focus of the faith would probably be redirected from the sacraments (which can no longer be validly performed) to scripture and preaching.
(I suppose Catholics could still achieve salvation by depending on baptism and perfect contrition, but going without Eucharist, Mass and confession is a big ask!)