It's a complete unknown and entirely open field of inquiry.

It's like asking about what types of books will be published in a 100 years from now, given we just invented the Gutenberg press.

In short, it will depend entirely upon the demand. This whole thing is about money, technology is secondary.

Presumably high transaction fees to offset the loss of direct reward for mining to keep the engine of PoW running so the network remains secure.

That's what is required for the network to remain secure, but there is no direct mechanism to cause that to happen. You can observe all the other blockchains with weak security as experiments: there is nothing preventing the security from just dropping to the point where it is easy to perform 51% attacks.

Absolutely but even as someone who is very much against crypto it’s such a massive hole that threatens miners directly I can’t see it going unaddressed. Or the whole thing collapses which suits me just fine.