Well...I think I understand the real-world reason why we never saw the Enterprise-E or another Sovereign-class ship in DS9 and the last episodes of VOY--it was the movie ship design at the time and was being reserved for the movies. Some studio exec probably thought it would make the movies less special if audiences could see the design in action on the small screen for free. I can't really argue that if that's the case, but from an "in-universe" perspective, it could be that there just weren't that many Sovereign-class ships around at the time--maybe it was only the Sovereign and the Enterprise for all we we know--and they simply were elsewhere during the Dominion War and equally unavailable later to welcome the Voyager home.
But since the Sovereign-class is no longer the movie ship design, it's free to appear in PIC, LD, and any other show set around the same time period, IMO.
But since the Sovereign-class is no longer the movie ship design, it's free to appear in PIC, LD, and any other show set around the same time period, IMO.