The hull was replaced because the previous one showed damage due to repeated stress cycling:

"This is shaping up as a rebuilding year for the nearly 11-year-old venture, based in Everett, Wash. The main task on the agenda is to build two new submersibles capable of diving as deep as 6,000 meters (3.7 miles), which is more than a mile deeper than the part of the North Atlantic ocean floor where the Titanic is resting.

OceanGate will take advantage of lessons learned during the construction of its carbon-hulled Titan submersible, which was originally built for Titanic journeys. Rush said tests that were conducted at the Deep Ocean Test Facility in Annapolis, Md., revealed that the Titan’s hull “showed signs of cyclic fatigue.” As a result, the hull’s depth rating was reduced to 3,000 meters.

“Not enough to get to the Titanic,” Rush said."

From:

https://www.geekwire.com/2020/oceangate-raises-18m-build-big...

From a link within that article:

> Because Titan was once known as Cyclops 2, the working titles for the new submersibles will be Cyclops 3 and 4.

So they lowered the depth rating of Titan back in 2020 .. but then continued to dive down to the Titanic with it?

With a new hull. But this was then also subjected to multiple dives but I'm not aware of any subsequent testing. Possibly if they had tested it defects would have shown up because that's pretty much the way this sort of structure responds to stress cycling. We'll never know unless a record of subsequent tests surfaces.

Isn't implosion like the one obvious danger of being deep underwater?

> I think they just didn't think there hull design would implode.

Even the diver that piloted it imagined at least one way how it could implode (window).