1) When the block reward is gone, miners will get paid from fee!
This problem will start arising in our lifetime, especially if after a few halvening cycles the price growth will start failing to compensate for them.
2) The immense cost of hosting a node if we double or triple the block size:
Blocksize increase will definitely happen sometime in the future, and it might result in higher block rewards because more tx per block = more fees. But would it alone make mining more sustainable? Probably not.
3) The lighting network dilemma
While the LN keeps "growing", it's also stalling. If you look from a BTC standpoint yeah, everything is fine, but LN was supposed to be for fast payments, moving instantly value, so the true metric here should be represented in the value locked, and if we look from the $ perspective (orange line) we're well below 2021 levels!!!!
LN is still in beta so people are reluctant to adopt it. And the fees are not painful enough to force people to start looking at LN. 10-15 sat/vB is not that much right now. If the minimum fee will consistently be $5 or more, than LN will start looking more attractive.
LN can justify $50 on chain fees, because they will be like a fee for opening and maintaining a payment card with instant zero fee transactions. If the channel will last for a year or two, that's not a bad deal.
But the worst possible scenario is low Bitcoin adoption with low on-chain activity and as the result low fee rewards in blocks - together with halvenings it would indeed knock a lot of miners out of the game and make the network less protected. Our last line of defense will be treating low confirmation counts as unreliable. If right now 1-3 confs is enough, 10-20 might become the new default requirement.