That is the argument many use against DPOS. It takes a super majority, 17 out of 21, to get anything done. That is quite a collusion to get. So the likelihood of that isnt great. Plus, those 21 can be voted out at any time if word got out. Unlike elsewhere, votes can be placed or removed at any time.

Also, DPOS is faring better than POW in decentralization. In POW the miners took over. Bitcoin is basically run by 5 or 6 mega-miners who control more than half power.

Finally, I would say the fact that the reward system is ever in operation helps with that issue. STEEM is consistently being doled out to users, 75% which goes to authors. Over time, we see more hands holding STEEM, which affects the possibility of Witness make up if people do vote.

It is not an easy question because none of us can see the future. One interesting note is Ethereum is going moving to POS after using POW...Is that confirmation for POS?