the "problem" is TOKEN DISTRIBUTION.

(IFF) 20 accounts control 51% of the active votes (THEN) decentralization is dead (which is fine, just stop pretending)

however,

if someone can place a witness within the top 20, it can block any NEW changes to the blockchain.

generally, people shouldn't be able to constantly change the rules in their own favor - the top 20 can freeze and or outright steal accounts at will.