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Yes I think we could change that. Some other crypto currencies based on the Bitcoin code like Dogecoin, have no halving at all...
So I think theorically with a hard fork we could change the future block rewards, but since Bitcoin is famous for having a so called hard coded max supply of 21million,
your example of altcoins is good, but a better example could have been one of the fork coins that also copied the blockchain and changed the total supply, a fork coin like bitcoin-diamond.
also as far as i know 21 million is NOT hardcoded anywhere in bitcoin's code! there is a lot of other things in place (the block rewards, and halvings) that ensure the cap is not higher than 21 million.