At the moment the miners for a block are awarded 12.5 BTC. Every 4 years this value will fall by half. For example, let's imagine that the prize value is 1.5 BTC for each block and that BTC's value against the dollar does not increase. In such a case, mining will not be very profitable.
At the same time, miners are also benefiting from the value of the fee. This time the fee values will increase. Then people will have to pay a high fee for bitcoin transfer. The biggest advantage of the Bitcoin system against banking systems was that they could shop without paying commissions to any brokerage house. If this happens, can bitcoin preference be reduced?
If someone stops mining, then the difficulty goes down, this means the ones that are still mining get a benefit from continuing mining. So if someone leaves, the system automatically self regulates in a way that it creates an incentive for the rest of people to keep mining and for other people to join in as the difficulty goes down. There is always profit to be made by mining BTC.