i think it was not the whole hull but one of the materials, the hull was made of that had expired. well, carbon fibre has its strenght when pulled, but when pushing it bends. but if one uses resin on the fibre, then it gets some strenght when pushed too. similar to steel and concrete, while steel can really be pulled a lot, concrete is way better when pushed than steel. steel is quite stable when pushed too, but thats not its main strength. i think the resin was what really held the pressure in the sub, not the carbon fibre, but with this i only have that dangerous type of half-knowledge i'ld have to bring to expert level before doing something stupid (like depending on that to be fully true without really knowing).

in general things often last longer than their expected "minimum" to be used without concern. but in practice one would have to test for damage or if its worn out (like its done with airplane parts at fixed intervals) even without using materials of bad quality. but that was AFAIK what oceangate's management decided to explicitly NOT check the sub for - despite internal demands to do so.

i would not say its not possible to build a secure pressure hull out of carbon fibre, or out of carbon fibre of not the best quality, or a hull of a different shape than a sphere, or a hull out of different materials with different bending behaviors under pressure, or when such components are "glued" together on the edges that do the different bending, but ALL of this at the same time and without even checking at least after a new maximum depth was reached? not to mention crackling sounds after which heared one would want to double check. Even the wright brothers seemed more cautious to me.

today one would at least get some wear level statistics with unmanned vehicles in a slightly deeper than intended depth to have security margins and afterwards throughout checks for the parts that are important, single points of failures or are one of the proudly new developed.