This is great news, and there won't be too much duplication for me. I've mostly been listening to Air albums like Air Song, Air Time, and Live Air, although I do have Air Lore on Vinyl. I just acquired Threadgill's Carry the Day and Where's Your Cup within the last couple of years, but I somehow managed to misplace the former CD, so I guess this set will remedy that.
As for the 20-year span, it's a little unusual but nothing to get bent out of shape over. Mosaic is doing what they need to do to make some of this music more available, and I'm grateful for that. I know that such chronological jumps can be jarring on a single-disc compilation, but who would listen to a box set as big as this straight through? Just take into account the recording date of the material on each disc as you take it out to listen to it.