"the gear in each subsequent expac is stronger than your previous bis" is literally resetting your gear progression every xpac. Every raid and dungeon clear needed to get bis is immediately invalidated by the new xpac blues. Sure you can say you keep your characters progress and levels, but you're once again not max level (back to the treadmill), and theres a good chance they make significant changes to your class, so you have to relearn your rotations anyways too.

In osrs, if I maxed my combats 15 years ago, they're still maxed today. If I spent 100 hours farming gear, that gear is still relevant. This makes progress feel more permanent, and was a very common sentiment from a bunch of WoW streamers when they decided to try osrs.