Chinese miners can decide tomorrow to change code to produce more than 21 million coins if they think their profit is not enough.
Wrong. 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.
I am not aware if there was an attempt to fork to a continuous 50 coin reward. If there was one, the hashing power was not concentrated in 3 mining farms at the time of the first Bitcoin halving, this made all the difference. The strong decentralized fork won.
"Miners will go where the economic majority is" - it's a theory based on what they have done in the early years of Bitcoin. Powerful competition would not matter that much if Bitcoin hashing was dispersed as it was at the time of the first Bitcoin halving. The problem is conditions that define faith in Bitcoin have been changing to notice for anyone paying attention. We can no longer make a clear-cut conclusion that miners haven't turned into dictatorship class. Sure they don't exercise their full power yet, but this is temporary while they are content with profits and there is no competing decentralized Bitcoin fork to challenge them this time.