I think the design was flawed from the start, proper stress testing would have revealed it. From what I understand they basically sent it down a few times and said all good, we're done.
The sub did have titanium front and rear bulkheads. If their goal was to make it cheap and light, they might have done better hatching together a train of CF spheres. A cylinder is not strong enough.
Though to be fair, even the best design with the most rigorous testing can fail catastrophically. If that weren't the case space flight would carry no risk. And space is easier to deal with than the pressure at 4km ocean depth. Still that doesn't change my opinion of Rush, he was a hack.