From 2099 till 2140 (when the last satoshis will be mined) there will only be one bitcoin mined. By the end of the century, supply is virtually fixed and limited to the maximum of 21 million. For the following forty years, miners will be mining only one bitcoin...
With most transactions (and most tx fees) being forced onto second layers, it will be interesting to see how this impacts miner incentive and their desire to secure the blockchain.
My best guess is that by that time, Bitcoin will have become a minority player in the space due to its inability to evolve.
This might be considered sacrilege by many, but will we even be seeing Bitcoin on the same algorithm by then? And no I'm not necessarily talking about a move away from PoW, but could we be seeing technological advances that either make PoW "easier" to achieve while still staying ahead of whatever threat it is people are coming up with (like quantum computing?). Or will tech branches simply veer away somewhere else?
Bitcoin won't be what it is today for sure. But hard to predict anything when even the very fundamentals of economy, money and society might have changed then.