Well, sure--Blue Note for one, to make a case quite close to home. Where did most of the posters on this board come from? Of course you buy for the music, but I don't think you can deny that a # of the people who post here certainly "trust" the Blue Note label of yore. And Mosaic, IMO, has very much built upon that sort of identity/brand/label loyalty.
I agree w/most of what you're saying and don't think that ultimately this is a big deal, but I would certainly define Mosaic as a "label"--even if they do only reissues. A label in the sense of a record/business company purveying a certain brand of music. And yeah, Blue Note recorded a diversity of artists, but there are a lot of folks on this board unhappy with the current incarnation of the label for the direction they're taking and the artists they're signing.
Like I said, no big deal to me, as long as they keep putting out the classic stuff... and yeah, the timeframe inevitably stretches and has to... I mean, hell, the Tony Williams set covers recordings made after Mosaic was launched. But that doesn't have much to do with what gets defined as worthy jazz... we have yet to see a "sweet-music" band set from Mosaic, even though they do lots of early jazz. There's a lot of stuff from the 1980s and 1990s that I'd like to see Mosaic get around to putting out, but none of it emanating from the "contemporary" purview.