It sounds like they could use some improvements, 5 people are probably dead now. Out of how many trips in that submersible? I don’t know, just wondering, but it can’t be that many. Certainly not comparable to commercial airlines. So that’s a pretty high failure rate. Being able to open the vessel from the inside in an emergency probably wouldn’t have saved them, it’s just the first thing that is too much of a red flag for me to personally ever step foot in that thing.

I also don’t even understand why that vessel needs to be manned at all. With today’s high def cameras and displays, including VR, they could outfit an unmanned vessel with high def cameras everywhere, a fiber optic tether to the surface to carry the high def signal (or even wireless to some degree if they could make that work), and the operator could sit in a control station either on the boat at the surface or streamed over the internet from home, controlling the vessel however he wants, with high def screens to simulate the viewing experience of being in the vessel. I’ll bet it would get you 99% of the way there without having to risk your life. It’s not like anyone is getting out of the thing to swim around the titanic in scuba gear anyway.

There is no hint that its probable demise had anything to do with hatch design so far.

This is a recreational dive, there's absolutely no need for it as for most recreational activities.

There are quite a few other manned submersibles designed for extreme depth that do allow for the hatch to be opened from the inside, in fact that's most of them. Alvin's design would be a nice starting point. It can be opened both from the outside and from the inside.

As for what the probable cause is: we don't know, we may never know. But with a device that was apparently designed by another party who declared it unfit for purpose anything goes.