Gear power being ephemeral can be fine. Like I said, I'm not totally against seasons, but give me a compelling reason and some kind of payoff to want to progress it beyond higher numbers. Yes, that can be cosmetics, trophies, unique experiences, etc.
Challenge mode gear being removed was/is dumb. Adding new cosmetics doesn't mean old ones need to go away. Content should be additive. If in the case it's FOMO, it can come back in a limited time thing sporatically
Its fine that theres stuff new players can't immediately do. Wandering into western plaguelands as a lvl 11 undead mage from Tirisfal, made me really want to level up and fight stuff in western plaguelands. It's good. Gives them something to look forward to and a reason to want to progress. That's the point. If it's seasonal stuff it wouldn't be walled off by a year or 2 or whatever. If it's too high of a grind, it can be adjusted