OK, let's say that a pro BTC gov holds 20% of the supply. The next president to come in is a dollar maximalist and they dump it. The price goes to zero.

It just becomes less attractive if most of the supply is in the hands of a bad actor.

BTW, doing the math on this its pretty much impossible or would take 20-30 years. 

Please explain to me how this is bad for decentralization given the current consensus mechanism.

In my understanding, all a whale could do is try to manipulate the price, down or up. Since there would be no economic reason to drive it down, what's the problem?