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?
Bitcoin is a POW blockchain and not a POS blockchain. Buying any amount of Bitcoin never gives you any power. In a POS network this could be a problem, but here no.
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?
Based on the title of your post, miners are the ones keeping the network safe from 51% attack, they are keeping the network secured. They also validate transactions and create new blocks by solving the cryptographic puzzle. Currently, miners are not making bitcoin centralized, and I don't think that bitcoin will lose her decentralized nature in future. Only if the government start controlling mining pools to tell them which transaction that they add to the block chain or not.
Miners have an economic incentive to operate according to the rules of the network and ideals behind the network. There is a huge incentive to never do a 51% attack as one will have more profit by behaving correctly. Users hold the power of the Bitcoin network no matter what business people want to believe. Tomorrow all miners could be made bankrupt with an emergency fork. Which miner would want to risk that?