Tangent, but still related to the OP, I've always felt TNG would have worked as a revolving door "Law & Order" style show. Just rotate in new cast members as older ones want to depart. Promote Riker to Captain. Give Data his own ship. Promote LaForge. Transfer Worf. The drama becomes more about the mission, which TNG kind of already had going for it.
I'm not saying that the cast members are completely interchangeable, but I could really see this format working for TNG. Rather than spinoffs, your show is
your spinoff. You reinvent it with new main characters. Obviously, this also wouldn't prevent DS9 or VGR from happening. Look at how many "Law & Order"s there are.