Delegations should be done with care, and align with your own ideas. As long as you watch what happens with your delegation, I don't think it hurts decentralization.

Witnesses don't have to be active on the platform necessarily, their job is keeping the chain secure. It's hard to judge someone who doesn't show a presence though. If you know someone who runs a witness node because he likes the tech or the idea of decentralization but doesn't blog, you should definitely vote for him!