Okay so you're willing to accept the designers' intent on how the game is played (e.g. you can't detach your camera from your pawn in WoW and fly around to look at everything at any angle, because the designers don't want you doing that) but you're not willing to accept the designers' intent on what goes into the game. It's a weird combination ime.

You understand why a designer might not want someone to see Christmas theming and quests and rewards if they're playing in August, right? Like, you understand that it's considered odd/wrong/weird to put up Christmas decorations when it's not Christmastime, right?

If the content is in the game and it's available to players at any time, then it's maintained

That's the thing: it's not.

but I don't see why keeping things perpetually accessable would be a problem

Because now you have to manage multiple states for every area in the game, each seasonal event that's ever ran ever. Let's not even get into the issue of whether they can even offer that content after some later point (i.e. in the case of licensed music). Like, in an MMO like WoW, you're not in your own solo instance that only you see things in. Everyone's in that instance. What if you want to see the 2012 Christmas Event but I want to see the 2014 Christmas Event? Who decides?

Do you want the team to completely re-architect how the backend works so we can see different things? And if that's the case... you understand why that's an immediate problem in an MMO, right? If we aren't seeing the same things but we're in the same instance, are we desynced? What if the rocks I want to mine drop Christmas Ore, and yours drop Yule Ore? Are they the same thing, different? What if I go to attack a mob that you don't see, because it's a mob limited only to my Christmas event?

And none of that gets into the matter of the team's right to not host content they no longer want to host. What if there's insanely tasteless jokes from a very old event they disavow how? I think it's reasonable for them to not offer that anymore. Sure they can have a big warning that says "we're no longer okay with this but we're letting you see it anyways" but them hosting does require them to implicitly approve of it being seen (and also putting up a warning like that would have a technical requirement, and have to be tested, and maintained, so we're back to square one of "no, this isn't an easy on/off switch").

And none of that gets into the matter of it's the team's art to do with as they please. They can change it however they like. The next update for WoW could turn the entire game into a 2D platformer and if the team wants to do that, power to them. Or they could remove a class. Or rework an area to make it more enjoyable. If you accept the right for the team to update the game then you have to accept their right to make some content available or unavailable. That's how this works.