Say they do.
First, in the process, they'd push the price of bitcoins way up. And this is while they're buying. So they'd be buying high -- not a very good strategy. Every bitcoin holder would be quite grateful to them.
Now, they start taking over control of bitcoin. Let's just assume somehow they could do this. Well, if they make bitcoin more valuable, good on them. If that makes it less useful for some things that are important to people, that's no problem, they can just create another system or use some other existing system like bitcoin cash or litecoin.
If they crash the value of bitcoin because people don't want a centralized system, well then they just bankrupted themselves. They bought all these bitcoins high and now they're worth much less. And everyone else can still move to another system.
So it just seems like a way to waste a lot of money, unless they can make bitcoin better and more useful. In that case, awesome. Either way, all the people who sold bitcoins to them as they bid the price way up will have plenty of money to build a new system that they have no influence over.