I'm just an armchair observer here, but from what I can tell from the discussions, carbon fiber certainly can be an appropriate material for a submarine, as long as it's only used for one dive.
The reason it's a useless terrible material for a sub is that until it survives the first dive, you don't know if it has a hidden flaw.
And after the first dive, even if it survives, you know that 100% that it does.
Weight is not nearly the problem it is for a sub as it is for spacecraft or aircraft.
The problem wasn't safety protocols or XBox controllers or any of that.
It was conflation of "big idea people"[1] with real engineers.
As I said, I'm an armchair observer here, not a materials scientist.
However, there are a lot of things that are absolutely designed and built to be single-use, especially in fields like space, where rocket engines have been single-use until very recently.
As for weight, I think the reason that was important was because they didn't have a big enough ship that could operate a crane and haul the submersible out of the water. Really, the whole operation was a bad idea: this kind of exploration is expensive as hell, and the only way to cut costs (with current tech) is to do really dangerous stuff.
And after the first dive, even if it survives, you know that 100% that it does.
Weight is not nearly the problem it is for a sub as it is for spacecraft or aircraft. The problem wasn't safety protocols or XBox controllers or any of that. It was conflation of "big idea people"[1] with real engineers.
[1] see Jobs, Musk, etc.