Seeing big companies buying billions+ dollars of bitcoin, isn't it becoming too centralized. Could they centralize it if they own a lot of it's portion.
How can we defend against this process?
No, Bitcoin is never going to be centralized. Institutional investors who have the capacity to buy that much bitcoins can only manipulate the price of bitcoin with their massive buy/sell, but don't have that capacity to act as a central authority.
As for miners, they also are not in the position to make Bitcoin centralized. Miners have some crucial responsibilities they perform to keep the Bitcoin network going and it never contradicts the decentralized status of Bitcoin. You should read up more about miners to get clear knowledge of who they really are and what they do.