Nogami

Subs have dual hulls so they can fill the void between them with water to sink, or air to float, but they don’t dive anywhere near as deep. Most regular naval subs operate above 800 meters. They only have to be below the surface of the water for stealth, not at the bottom of the ocean.

This craft was a single hull design as far as I’m aware, designed with a rigid hull to try and counteract the pressure at the bottom.

Also when carbon fibre is used on other applications such as in planes and such. There is always something to be learned about failure modes which can be used to make future applications safer.

For instance, in this case where exactly did the failure point happen? Was it at at the bonding area, one of the end caps, or in the middle of the tube, etc.

The government really has no business dictating how manufacturers design their products. If people don't like phones without user-replaceable batteries, they won't buy them, will they?

The days of replacing empty phone batteries rather than recharging them are long-dead. I agree that replacing a modern smartphone battery, even in a sealed unit is stupid-easy. With the proper instructions and tools, my mom could do it quite easily. I've done it myself and it was at most a 15 minute job.