The game that handles this madness best imho is Guild Wars 2. There is no power decrease or obsolescence of items whenever a new expansion drops. There’s just new content. Pick RIGHT up where you left off. Even with the gear treadmill dead, you still have players playing through the new content. Why? Because it’s new content. Simple enough a reason. The best part is that the previous expansions are never trivialized. If you or your guild ever gets bored of the current expansion, you can just move back to an older one and work on progression there.
Imagine being sick of BfA, and the guild decides “Going back to ICC sounds so much fun. I haven’t been there in years.” Guildy 2 says “I wasn’t playing at the time, I’d love to try the raid out.” And so you go to ICC and try to reach the Lich King, picking up some completely relevant loot while you’re at it.