I'd like to take a moment to talk to you about ice cre... I mean, expansion packs. And I don't just mean WoW expansion packs, I mean expansions in general. For almost every computer game on the planet that has expansion packs, those expansions have followed two rules.
First, installing the expansion packs does not remove your ability to play the original/earlier version(s) of the game. If you install the "Warlords" and "Beyond the Sword" expansion packs for Civilization IV, and then decide that you're in a mood to play "classic" Civilization IV, you don't have to uninstall and reinstall. All three versions of the game coexist on your hard drive and you can fire up any version at any time. If you have Diablo II and Lord of Destruction, and you want to create a new character, you're given the choice of creating a Classic D2 character or an expansion character, and to sweeten the deal, there's an option to convert Classic characters to expansion characters at any time.
The second, and far more important rule, is that players with the expansion packs have always been separated from those without. People who bought Warlords but not BtS never found themselves getting their asses kicked in multiplayer by someone who had access to spies and corporations. Tiberian Sun players without Firestorm don't have to worry about getting roflstomped by players with Firestorm. AvP 2 players without Primal Hunt, who play as either Predators or Aliens, don't have to worry about whether the human player that they're stalking has a Marine motion tracker or a Corporate motion tracker. Diablo II expansion characters cannot join games created by D2 Classic characters. Starcraft Classic players don't get their asses kicked by players with access to Brood War units.
Both of these rules were made and followed for damn good reasons. The reasons for the second rule are especially relevant here: players should never feel compelled to choose between buying an expansion pack and being at an unfair disadvantage. If players who buy an expansion pack are allowed to compete with players who don't buy it, then the game basically becomes pay-to-win.
Although the various expansion packs for World of Warcraft have all made wildly different mistakes from each other, there are two mistakes that they all have in common: one, they were forced on everyone without regard to who bought them and who didn't. Second, players were all thrown together on the same servers regardless of who bought the expansions and who didn't. I do not need to reiterate the negative consequences of these decisions. How much complaining do you think there would have been about Burning Crusade if the people who bought it were forced to transfer to BC-only servers, and the people who didn't buy it got to stay on 1.12 servers where their hard-won T3 armor and Atiesh were still the best gear in the game? How many people would have complained about the Shattering if they still had their choice between the Old World and the new? Would I complain about how Warlords ruined professions, the storyline, Upper Blackrock spire, and everything else in the game, if I was still chilling with some drunk-ass pandas on the Timeless Isle? Would ANYONE complain about ANYTHING in this game if they could create new characters in any version of the game they wanted? Of course they would, because this is the internet and people will always find something to complain about. But the tone would be different because each new expansion's mistakes wouldn't matter. We'd discuss which expansions were best or worst in the same casual tone we'd use to discuss our favorite Linux flavors, rather than the apocalyptic tone with which we discuss forced upgrades to Windows 10. A bad expansion would cost you nothing more than $60 if you bought it, and nothing at all if you didn't. We wouldn't argue over which expansion or feature "killed" WoW, because bad expansions wouldn't be capable of killing WoW. Splitting us up by expansion pack literally makes every expansion better.