Say, at some point big banks and hedge funds make an agreement with core developers to remove the limitation of 21M and have the foundation vote for this?
Is it possible in theory and on practice?
It is possible but only with the consensus of the network this would then result in a Fork
However I do not believe it will work in practice although Bitcoin has been forked once before by accident
https://bitcoin.org/en/alert/2013-03-11-chain-forkSimply put it would be very complex to do so without consensus since it requires all users to create a single block chain compatible with all bitcoin software.