Bitcoin's Security Budget.
Without fees going up, and eventually the inflation falling to 0 - bitcoin's security decreases.
Something like tail emission which Monero has, would have made bitcoin network more secure in the long term.
Discussion
Don't you think the fees will go up on the primary layer, since almost all transactions will happen on layers 2 and 3? Even if on chain fees go up quite a lot, it won't have much impact on lightning fees, as the lightning nodes compete for lower fees.
I think it will take a way longer time for this to become an issue than you might think. Like a margin of decades and decades.