How much hashpower do we need to keep the blockchain secure? Maybe it's less than what we have today, though I suspect it's more. (ASIC mining reduces the risk of 51% attacks because there's no huge mass of "dark silicon" that could be temporarily redirected from running Slack or Monster Hunter to mount a 51% attack; the vast majority of hashpower that has been manufactured so far is busily mining away to support the blockchain.)

I'm not convinced that higher transaction fees would be a huge disaster. Maybe Lightning will work out (El Salvador seems to be betting heavily on it), or maybe people will use "custodial wallets" like LocalBitcoins, or simply run up a tab at the bar; with any of these approaches, relegating the public blockchain to weekly, monthly, or trimestral settlements for most people would be a mild inconvenience rather than a disaster.