The difference is, there are systems in WoW that you spend the whole expansion grinding, just to be relevant at endgame, and then have all that progress taken away when the new expansion releases.

Palace, BLU and Diadem are nothing like this in any way.

Palace is relevant to everyone and can be done today - to level. You never lose that experience once you have it. Palace hasn't been "abandoned". It's finished. It's level 1-60 content. The comparison here would be if they removed palace after a new expansion came out and everything you got from it just disappeared.
BLU is side content that is still relevant for fun - it's completely seperate to the main game and you don't have to do it to be relevant.

Relic weapons are somewhat closer to this system, but even then, they are designed for the casuals with other ways of getting weapons. Savage = hard but quick. Relic = slow but easy. That's the point.

You could compare Eureka or Bozja if that was the only way to get an endgame weapon, but it's not.

I don't know why OP is deliberately being disingenuous to push some narrative, but tbh I have seen a lot more of these sorts of threads recently since the WoW exodus. It feels like former WoW players are trying to pick apart anything at all to try and justify their previous "wasted" time and affection for WoW. The peak of this was the person who said dungeons were bad for new players because there isn't a pop up window telling you all the boss mechanics.