The problems here are just several fold: maybe the door won't help you at depth, but there's a pretty wide envelope of conditions in which a fast escape hatch is the difference between surviving a fire and burning alive.
The CEO of OceanGate said he "learned the lessons of aviation", but this is obviously BS if he's locking himself in a capsule with flammable equipment, piping oxygen in, and just hoping it doesn't burn. That's a recipe for disaster we know how to avoid.
If we’re talking worst mistake I think the lack of any form of communications equipment is it.
Hell, have a 4km long cable. Even if you can’t communicate because the sub is on fire or imploded you can drag them back up.
Of course it’s not nice if your sub implodes, but that’s kind of a requirement for using it in the first place.
Much more concerning are the potential flaws in the carbon fiber hull, as disclosed during the David Lochridge lawsuit (mentioned in another thread), the rumoured lack of surface radios, the limited communication with the surface, the lack of any exits, and even the use of carbon fiber (which is strong in tension, good for high pressure tanks, but weak in compression -- not what I'd want for a submarine under massive compression)