We can speculate, but we don't know. Unless you know a time traveler from the future, we don't have a definite answer.
However, getting to rely solely on tx fees happens gradually, which each new halving, so we can try to get an idea. On the other hand, the final step will be far in the future, hence you should not be that much concerned.
For now the things go well, so there's nothing to fix/mitigate. I hope that you agree on that.
Then yes, if it gets unprofitable, some miners will be stopped. It has happened before. The network does adjust in such cases and the mining will be more profitable for the rest.
Keep in mind that there are already miners that run using electricity they produce. This (after all the equipment is paid for) means pure profit, something that may work just fine even if the tx fees remain small. Then the others will turn off, these businesses will prevail, keeping Bitcoin network running.
Then, as already said, I expect that some entities that use Bitcoin as reserve asset (now big companies, in the future maybe countries too) will also mine Bitcoin, not for profit, instead simply to do their part in securing the network.
I wouldn't really call it a prediction that requires us to be a time traveler. I mean the price is out there, without the mining of bitcoin we won't be having 900+ million dollars worth of bitcoin, but that was double and now in half after the 2020 may halving and it's still quite secure. No matter what happens, the price of bitcoin makes sure that the miners do end up with a profit, and that's the only thing that matters.
So this means, we do not need to calculate anything, the question of "will transaction fees be enough" is simple, if it is profitable then it will be, if not then it will become profitable because difficulty will drop.