It depends on what you mean is truly decentralized. If all witnesses come together and fork and make sure there is no whale with a bigger share than x and each witness runs a different witness code (different language different operation system) I'd say it's truly decentralized.
Doesn't have to be thousands of people, as long as it's an elected group by thousands of people and everyone could lose their place. Although I believe witness votes should also decay after a year for that.