Decentralization was the main purpose that “Nakamoto Satoshi” created Bitcoin, at least that’s what he claimed and most people accept.
An infamous case when a cryptocurrency suddenly became highly centralized is Steem. When Justin Sun took over Steemit Inc., he confiscated tokens of key community members who opposed this acquisition. Yup, when a cryptocurrency (any currency in general) is highly centralized, the security of your assets is basically non-existent. Whoever possesses the 51% power can take coins from whichever account he desires.