I do see your points, where the chain itself is the cause of inconvenience or failure scenarios. Since the conventional bicycle is almost the textbook definition of minimal design, there are -- and will always ever be -- tradeoffs to make, and I concede that efficiency isn't always everyone's top priority for their bicycle.
While I cannot discount that this chainless system would ameliorate the situations you described, I can offer some alternatives which can or already do exist today, that could probably be explored and improved over what this chainless system delivers.
chain stretch, chain damage, and mud I go through 2 chains per season plus a couple trail-side repairs
The front chainring is also a big point of contact on mountain bikes, removing that could improve clearance with the right design
The older style of shaft drives in lieu of a chain could be maintenance free, and damage resistant. Efficiency is lower, and roadside repair is much harder, but efficiency is already lowered when in messy terrain, and if it's fully sealed, it might last a very long time anyway. Reviving this concept is something I could reasonably see.
offers a much wider power band than gears and a chain
Having discrete gear ratios will indeed always be a limiting factor when dealing with variable speeds, but there already exists a CVT (continuous variable transmission) for bicycles, where torque and speed can be traded in non-stairstep fashion. That said, no one makes a CVT with a built in ebike motor..... yet.
You could also optimize crank length for clearance instead of torque
I think I understand what you mean here, in that cranks today draw a larger circle to accommodate human physiology, but that also increases the risk of a pedal striking the road, or rocks adjacent to a trail. But I don't see how this chainless system can change the crank circle diameter on-the-fly. And I don't envision people wanting that very often, unless they want small diameter on trails and then want to bike home on the roads by switching to a large diameter.
regenerative braking
I think this already exists for ebikes: https://radpowerbikes.zendesk.com/hc/en-us/articles/360045171734-Riding-Rad-with-regenerative-braking
I want to be clear: I'm not trying to shoot down the idea of chainless systems out-of-hand. But rather, these other solutions to your scenarios are more grounded in existing technology, or are waiting to be further refined before mass adoption, and likely will cost a lot less.
