Unnecessary, the far future of bitcoin mining will be integration at the source of energy production or into every electrical appliance/industrial machine in the world. Look at heatbit as an example of this. The space heater uses no excess electricity but mines bitcoin as part of its operation. It is expensive now compared to other space heaters and likely doesn't mine enough to cover that difference but in 10 years it could probably be made at a similar cost to a regular space heater and even if it only throws off a small amount of bitcoin it is still better than a space heater that doesn't mine. There is also the idea of alturistic mining. I think many bitcoiners, corporate entities, potentially governments with a significant portion of their net worth in bitcoin will mine to preserve the network even if it is at a loss. This is similar to a tail emission but selling the idea of mining as noble and an insurance on your stack is much more palatable than a change in supply.

I am not worried about the security budget 20 plus years from now. The market will sort it out and if it doesn't Bitcoin has likely failed.