After MOP remix is almost certain that WOD and beyond wil return with their remixes too. probably 2 year cycles so why would someone farm say, HFC raid for mythic archokminde mount with 1% drop rate when you can eventually 100% buy it for a days worth of work.
This also applies to transmog sets, i mean why go farm xmog now, you can essentially just buy it when the remixes come out.
Because there’s no guarantee that they will.
Its very likely. But not a forgone conclusion.
Never underestimate Blizzard’s appetite for completely ignoring a function after it’s finished.
There could be a couple of reasons:
having fun?
At the end …why you play this game ?
90% of players are using the same transmo for 5-10 years … In my guild 10 at least of my close friends are using the same transmo since draenor . I’m using t3 old naxx since transmo was added…
Why would you bother collecting something that you would never use ??
How do you know ? Illuminate me… 
Even IF one assumes that remixes will come back (not guaranteed) and even IF one assumes that they will happen on a 2 year cycle (not guaranteed), it will take how long to get through all the expansions???
I don’t know about you, but I’m not waiting for 8 (number of expansions without MoP) * 2 (years), so 16 years until all those expansions cycle through. And on top of it, we don’t know which expansion will be when, which will be first and which will be last. I’d almost bet that something like Legion or WoD would be the next (if the remixes happen again at all), but after those?
How long are you willing to wait instead of playing and enjoying the game now? I don’t even know if I’ll be playing this game in 5 years time, much less 16.
i think its obvious that very few people are bothering farming old content, or have the old sets and blizzard want them to become more accessible at this point.
i mean, isnt it obvious? do you have a reason they specifically chose Mop? It seems kinda obvious, after cata classic, they wont release MOP classic, they wil just go back to vanilla and re release that
Sure, if you want to wait a decade for ‘current’ content to be remixed go ahead. 
There’s no guarantee that they’ll do every expansion as a remix variant. Even if they do, it could be years. Besides, every mount or transmog you earn in retail that you really want to acquire is less coins/time spent farming it if and when the remix you want swings around.
You don’t have to farm as much bronze if you own some of it already.
Win Win.
Obvious would be quoting a blue post saying that. Just to mention…
Legion might work as remix…
But i cant really see other expansions doing as well as mop.
Mop had the content for it.
The content is a bit rep farming, some achievement and then raid farming. Any expansion could do that IMO.
WoD would be nice next.
See i dont feel wod would be good in that sense, while there is reps to farm, to me wod was like a push expansion to modernise wow, it failed in multiple ways imo, others will see that differently.
Mop for what it was, was still very much riding on the coattails of vanilla and tbc so there was more depth to design.
Wod things started getting simplified, legion being the only one after which kind of reverted back.
I don’t see the issue, it had fantastic raids, and the reps are much like any other expansion. Remix isn’t some great in depth visit to an expansion.
Beeecause I want it now. Any further remixes just finish my collection.
As much as i hate to say this if its just about reps, the best expansion to do a remix of is probably BFA
The bulk of my activity in Remix is raids. I did do the Remix achievements but many don’t bother with them at all. They don’t give anything.
The raids are where you do the bronze grinds. So to me that is the key point.
Everyone is different.
I think WoD also has a nice selection of world/open world mounts they can add to vendors as well as recolours that never made it to the game.
But you can like other expansions. There is no right or wrong.
I have enjoyed Remix more than I have S4.