Of course, this is hypothetical, as I don't believe that God would ever allow all bishops to die out. But, as you see in Ott, it's somewhat debated. From Ott's reference, however, could there be something missing, namely, that the Abbots in question also happened to have been consecrated bishops? We'd probably need to examine the Rite of installing an Abbot at the time, as it may have entailed at the same time a consecration. Abbots were considered to have some degree of jurisdiction within their monastery, and it would have been simple enough to confer episcopal consecration on any of the Abbots granted this privilege. So I'm not sure that we aren't missing a piece of the puzzle there.
You believe that god could allow bishops to die out but you believe that God wouldn't allow properly catholic hierarchs to die out. Wut?