Actually everything in Bitcoin can be changed, as long as there is the consensus for it.
The miners that have accumulated a lot of Bitcoin, while the whales that only wish Bitcoin price to be increasing, are they the one that will agree for a change in the total supply of Bitcoin to surpass 21 million, I think that is just not going to happen. I believe this is the last thing that can happen to bitcoin, even to be the last thing seem more of an understatement because the chance for such to happen is not realistic.