The expansion packs have non-linear increases in power. This goes beyond the tyranny of levels: the numbers become so large that it is only the same game because of overlapping mechanics. It is not so much an expansion pack as a sequel that imports your achievements and some trophies.
I tagged along as 3 level 80 guildmates took out the 1.1 million hit point Ragnaros. A difficult raid for 40 level 60s becomes somewhat difficult content for 3 level 80s. I don’t think the Priest had any heals smaller than my hit point total. Current endgame characters are literally orders of magnitude above the original endgame. It makes everything that came before superfluous, like having an artillery squad plus one guy throwing rocks as hard as he can.
Mixing the two is silly in the other direction as well. If it were possible to hit an “aggro everything in the dungeon” button, level 80s could hit it and solo most of the original dungeons all at once. If a level skull attacks you, there is no point in even resisting someone who does 10 times your damage with 10 times your hit points. I am on a PvP server, so anyone capped and bored can cut off a [patrol * sight distance] chunk of a contested zone at will. (Relatedly, although in WoW you also get a giant hammer while the kids get sticks.)
As I ponder being done and having won, the feeling that it is no longer the same game is very relevant. If it is some whole other game, there are lots of new games that are new, rather than the same thing with bigger numbers.