That's one reason Proof of Stake has benefits over proof of work. Steem Power is essentially Proof of Stake...you don't mine but you earn interest for holding. Full Proof of Stake also implies you're running a node on your computer which verifies transactions. So nothing is fully decentralized. And yeah they can change the game but that is called a fork. There will be at most 21 million Bitcoin. That's a parameter in the code and if someone changed that you'd have to fork off or copy and paste and it wouldn't really be Bitcoin anymore. The original Bitcoin would still survive unaffected as people would still want to mine that.