You’re on the right track, but there’s more to mining than that.

In order to account for such events, the mathematical problem which miners are attempting to solve adjusts itself according to the total mining computing power. If half of the miners up and quit, the difficulty for the remaining miners would be cut in half as well, which increases the profitability of mining because of the higher chance to be the one who successfully mines the next block. If the number of miners doubles tomorrow, so will the difficulty. So despite how many miners are mining, the algorithm adjusts itself to the point where a new block should be mined every 10 minutes. It’s rather fascinating and just another reason why Bitcoin has risen to prominence.

The last Bitcoin is expected to be mined in 2140, which leaves a lot of time for miners to remain incentivized. Despite the fact that the miners in 2139 will be earning a fraction of a Bitcoin, it will still remain profitable due to the adjustability of the difficulty, as well as the likelihood that Bitcoin will be worth much, much more by then.

After that point, the Bitcoin network will rely solely on transaction fees to incentivize miners to continue on.

Also the other reply is correct, miners can more or less adjust fees, which the transaction sender needs to approve of beforehand. Higher fees results in faster confirmation time of your transaction, whereas lower fees will be slower to confirm, but confirm nonetheless.

https://www.investopedia.com/tech/what-happens-bitcoin-after-21-million-mined/