The sub has already survived multiple dives to 4000 meters, and so far nobody has come out with reports of any apparent problems with the viewport. Maybe it failed from an impact or from accumulated fractures from all these pressure cycles, but I'm also completely willing to believe that the depth-rating for human occupancy has enough of a safety factor that going 3 times deeper doesn't lead to failure.

From the pre-accident reports about the sub it seems like hull integrity was the thing they actually cared about the most, with everything else being in an even worse state. My money would be on the sub drifting somewhere with no power or no control or whatever.