BTC code could be changed to allow >21m coinage.
Nope, but that's what the scammy Eth shills like you love to spam to try and pump Eth:
People already attempted this at the first Bitcoin halving by trying to fork to a continuous 50 block reward chain and it was a miserable failure. Even if you temporarily rented all hash power on earth to try and get temporary consensus, the economic majority would stay on the normal chain and miners would be forced to switch back and mine on it after their coup fails. Miners are forced to go where the economic majority is, not vice versa.