The problem here is that there's not good alternative to Lithium for now. LiFePO4 (commonly seen as LFP) chemistry is a little safer, but also less energy dense & heavier. It's unclear to me whether or not these downsides rule out that chemistry's use for bikes as space and weight can often be at a premium.

Agreed, and yes, this is a problem. I'm hoping for Lithium Titanate to become more affordable, I think it is one of the most promising candidates.

I forgot about that chemistry! I’m excited about it as well. It still seems to be unobtanium for my use-cases :)

You can get the cells but they are still too expensive and the form factor doesn't help either, but I'm hopeful that this will be resolved.

While true, this hasn't changed the fact that you can still weld plastic gas tanks.

Yes, but I would only do that for a car for which no tanks are available any more. If you can still get a NOS one then you probably should, unless the leak is very small and on the top of the tank (where it rarely is...).