That's what it was supposed to do but this time it went down and didn't come back up.

It would not follow a bathtub curve. As the Wikipedia article explains, you have to at least check the in-service products to get a bathtub curve. As this article [1] explains, they did not do that. Therefore, no bathtub curve guarantee.

For argument's sake, let's say it would follow the bathtub curve. You could summarize all the warnings they were given as basically saying that the wear out curve crosses the other two curves all the way on the left. You could also say that they took the bathtub curve for granted, not realizing that it's the result of deliberate work. So again, still no bathtub curve, no matter which way you look at it.

[1] https://abc7chicago.com/missing-titanic-sub-oceangate-lawsui...