I think it will last indefinitely. This is because fee revenue will continue to be a large positive number, and it’s more profitable to cooperate than to attack (as per the white paper). Fees alone would already be enough today, not counting expected increased fee revenue - Satoshi was only wrong about how quickly it would catch on.
How much do you think is enough? Is it an absolute number, or a percentage of market cap? Is it just copying the rate at which humans find gold? I’m not familiar with how Monero calculates its optimal tail emission, if you have any recommended reading.
BIP 300 sidechains are even more immutable than Lightning channels; they’re backed by months of confirmations on the main chain and cheating miners would waste their invested hashes. Even without LN, without sidechains, there will continue to be upgrades for more on-chain transactions per block. (Bulletproofs are epic btw)
I hope that after what happened to the BCH investors, we’ve all learned to not take anyone’s word as gospel. :-D