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.