The first-party solution, with all of the historical knowledge, employees, codebase, marketing budget, and so on and so forth, is going to be superior enough that these emulations will never gain significant traction anyway.
To me, it seems like an issue of legal departments getting out of control.
Dude, harsh.
As a developer, I wouldn't want someone taking the work I'd spent years to build, taking a slightly older copy and then iterating on it and selling it.
I'm not sure what moral justification there is for them to keep milking it, while preventing other people from effectively remixing their work - forever. I think of all the awesome things that could be done with WoW as a base if only Blizzard's profiteering wasn't the only relevant factor.
I've never played WoW nor do I know anyone who still does, so I'm out of touch. At what point (if any) were users forced to pay extra (a paid for update and/or increased subs) to continue to play the game? Can those without the expansions still play? That certainly seems to be a fair place to draw a line.
Having to accept changed game mechanics via free-but-compulsory patches is a much harder to define distinction as the matter is very subjective: what to some is a wonderful improvement may to others be a game breaking change.
It doesn't quite work out the way you're imagining. A WoW expansion does one thing that require you to buy the expansion: the level cap goes up (e.g. in Cataclysm the level cap was 80 if you were paid up through Wrath of the Lich King, but 85 if you had bought Cataclysm). An expansion may or may not also prevent accounts that haven't bought it from entering new zones.
But it also makes a bunch of other changes that you get whether you want them or not. Content may be removed from the game, like original Naxxramas. Game mechanics will shift around. Every raid below the level cap goes out of support -- it isn't possible to play supported raids without being at the most recent level cap, and it also isn't possible to play old raids the way they were when they were current, as mechanics changes are not evaluated for impact on unsupported raids.
There were so many conceptual problems with having multiple different expansions that weren't current that Blizzard has moved to the model of "you've either bought the current expansion, or you haven't" -- when one expansion releases, the previous one is now automatically rolled into everyone's account.
It's that the game changed beyond recognition over ten years.
The simplest analogy I could give is - imagine that you started out playing chess, and ten years later the game was chequers.
Each individual change was small, but they add up. You can still play 'something', but it's not what you bought, or perhaps more relevant what you invested significant time and emotional energy in to.
The core root of the game changes, mechanics, graphics, etc, and those with the expansion get additional content, but even if you don't buy the next expansion you can continue to play the game (assuming you don't mean to take advantage of the expansion content).