I don't think we can manipulate bitcoin price because its price will become bigger if the demand will rise higher. If more will need bitcoin so they will buy more of it so the economy will be better for digital currency. When the demand slows and the buying will decrease, price wil will also be affected it became more cheaper compared from previous with higher price. We are the profit makers of bitcoin price and it matters on your decision whether you sell or buy.
You are just explaining how the market works. Basically what happens is when someone's buying, the price increase. Otherwise, it will decrease. Nothing so special at all but that's what price pump-and-dumpers take advantage of. They pump the market, and spread rumors that it's a good buy, and when the price has increased enough, they will dump their coin. And since it will create panic selling, the price will decrease even more, then they can buy at a lower price again.
Now why does price manipulation doesn't happen often despite the fact that it's easy to do. One thing, the whales are not after that. They are in this for the long run. And they are thinking that it will be much better for the whole bitcoin community if no one is going to rape its market by doing pump and dump.
tl;dr: Yes, we can manipulate the bitcoin price if we have enough money but when you're at that position, you would realize that holding your coins will be much better.
Price manipulation usually happens in altcoins because the volume is much lesser and an average bitcoin user can do a pump and dump there.