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?
If Bitcoin was Proof Of Stake (POS) then yes, them owning more Bitcoin would make Bitcoin more centralized. But since it is Proof Of Work (POW) there is no need to worry about who owns how many Bitcoin since the amount owned does not represent anything else other than having a lot of Bitcoin.
Although that being said, it could be speculated that there is a certain meta danger that the government keeps pushing for POS and more regulation. I extremely doubt the Bitcoin community would agree to that, even if we were under pressure from various attack vectors.
In a single line: Bitcoin is safe, no need to worry.