You have a point. If there is a change in the supply and demand, bitcoin is affected and it is possible that by controlling this the volatility of bitcoin will be controlled. But I don't think that it would be that easy because volatility is one of the main features o bitcoin and this had made bitcoin reach its price now. Volatility is what attracted investors and if someone is able to control it through supply and demand therefore bitcoin is no longer decentralized.
Probably you can't controlled the volatility of bitcoin because it has already been a nature coin to decentralized currency. And yet there many investors who was likely in volatile due to the fact that no one investors would invest if the circumtance is stable because they have a lot of bitcoin in there own.