I'd say no one really. It's decentralized but as for it's value that could be determined from a large range of factors: miners and the market. On another point, however, while you technically can do whatever with your Bitcoins, if they're in an exchange or wallet or whatever that's been ceased, hacked, or shut down, your control tapers off as what happened with Mt. Gox and China exchanges right now.
it is decentralized indeed , but how about the 51% attack?do you think it will still make bitcoin uncontrollable?
i don't think so , at this point it was bitcoin weakness ,
and it could happened anytime , some people can control bitcoin.
I don't you have to worry about 51% attack at all. Mining farms nowadays cost billions of dollars. I don't think someone will invest much more than the best mining farms just to attack bitcoins. It would be much better if he's just going to mine bitcoins as well instead of attacking the network since attacking the network will decrease the price of bitcoins[1]. When bitcoin goes mainstream, there would be investors that will invest even more than the best mining farms of today
You seem to be confusing billions with millions
The whole market cap of Bitcoin is below 20B dollars at present, so there is no way some miner (or even all miners combined) could have invested that much in the mining equipment. Apart from that, there may still be a viable reason to perform (or make an attempt at performing) a 51% attack. If miners have already earned decent profits, they may collude to try this attack (basically, pretend to attack) to bring Bitcoin prices down to buy up cheap bitcoins (again). Besides, this is what we could well expect next from the Chinese authorities in the their ruthless fight with Bitcoin in China (and a lot more)
Oh, sorry.
Anyway, you were right that after these miners have made profit or at least have got their ROI back, they might attack the bitcoin for that strategy or maybe it was their ulterior motive right from the start. We don't see anyone doing it right now (because if so there would be floods of articles about the 51% attacker). Maybe the ones that have invested the most haven't got his ROI yet or simply mining more bitcoins would be a far better move than attacking the network to lower the price and buy at low price or they simply don't have any intention to attack bitcoins at all. Regardless, if someone big does 51% attack, other miners will be aware of these (along with other attacks) and use it as a part of their strategy.