I don't question that at least one of the spin-offs was mediocre
Only one was mediocre. Another one was almost completely awful.
But DS9 and TNG were as good as TOS ever was, and I can't understand the attitude, either. Bring on a series to run parallel with the current movies featuring a young nuCaptain Bateson and a nuBozeman (or whatever ship he'd be given in the nuVerse), is what I'd say, write it well
and keep it true to the spirit of Trek, and let's do this thing. Make it so.