Supply is hard wired and cannot be manipulated. Any attempt will be seen and rejected. Its impossible.Under this protocol it's limited but they can change the protocol and generate more supply.
But, if supply is truly limited... what happens when it's fully mined? From a tech perspective, if there isn't value in generation, it will fall to the side. Sure, the last 2.5M will take over a hundred years to mine but laws of decreasing returns suggests it will stop being mined before supply exhausts.
What happens when the blockchain has produce the total 21m supply? Nothing. Things continue as normal. Blocks mined ever 10 minutes. The only difference is that the block reward will stop and the miners are incentivized to keep mining and they are paid not in new coins but coins that came from transaction fees.