What makes bitcoin decentralized? I want to understand the defense mechanism of the Bitcoin price control.

A rich company can buy a large portion of Bitcoins from different people in different exchanges. Then they can hold on to those Bitcoins, essentially becoming a controller of the Bitcoin economy. A large number of companies can start doing that.

If that becomes the case, then will Bitcoin still be decentralized? Those companies with their fare share of the Bitcoins, can control the price or any other aspect of the economy.
I think you're not understanding what decentralized exactly means.

Sure, a company with a large stake can control the price ( up to a certain degree), but this does not mean that they control (the protocol) Bitcoin itself.

There's no central power which decides what transaction is good, or which one is bad, or who controls what.

which would still be the case even if company X had 90% of the bitcoin supply.