Is there some technical reason for this? I have only been able to find one tenor and alto brand that has a high G, the Sax Gourmet Super 400's and they're not really a well known brand, and likely a KHS made stencil. Why haven't Yamaha, Selmer Paris, Yani, Keilwerth etc done this yet? Altissimo G is the hardest note to hit on the horn, period. I can only hit it reliably on my Bueschers which seem to play the top tones much more easily than Conn, Selmer etc. It seems like a modern pro horn should have a high G, since the notes above G3 speak relatively easily and it's more about intonation with them, but the high G itself is very difficult on most horns and even very good players with a well developed altissimo range will trip over it sometimes. My Yani sop has a high G and it's very useful, I play it frequently and though I can hit a high G on my other sop too, not nearly as fast with false fingering, takes effort, and with a cold reed nearly impossible.
Just my thoughts on the matter. So many pros are playing altissimo register now it seems only natural that if they added the high G key that would eliminate that one speed bump between the palm keys and the top tones.