Interesting. Your basic contention is that a low block reward means huge capital investment in mining kit (as we see at the moment) ceases to be worthwhile. But a malicious player could make a huge investment, not for block rewards, but to make a 51% attack. But then you need to show how he could actually profit from such an attack. (Simply saying by 'double spend' doesn't cut the mustard here - I've yet to see anyone lay out how a big double spend (and it would need to be big to justify the investment) could in the real world be sure to pay off.)

Bigger blocks - maybe. But they'd need to be huge. Perhaps this is an argument for smaller blocks, to guarantee transaction fees are hundreds of dollars. So all transactions move off chain (to Lightning's successor's successor's successor...), and only very high value settlement transactions remain on chain.

I can't help feeling that market forces will work out the solution and this won't actually ever be a problem. Or a technical solution that looks for 51% attacks (looks for orphans maybe) and protects the network accordingly. Need thought - we have 30, 50 or a hundred years to think this one through...