I think this is one thing people tend to consider a lot.
The past is not now? Runs are already cheaper than they could have been due to a lot of MoP sellers trying to sell in WoD and struggling. (This coming from someone who was trying to competitively price early in the expansion) Runs are much harder to 4man than even 3manning before.
For reference, think about a run like this:
https://www.youtube.com/watch?v=GmpDw8djHnE
Extremely optimized, and whatnot, I understand. - However it's also a matter that instead of massive overkill, runs require a larger group effort to really optimize.
So taking 1, or especially 2 people out, makes it a hell of a lot harder on the people carrying.
MoP CMs, especially by the end, were simply easier. From start to finish.
Point blank though, if you don't want a carry, don't buy one. I pugged my way from start to finish on multiple toons both expansions. - I also sold carries both expansions after pugging on toons I had it already done on.
It's not necessarily easy or fun getting your first sets of gold, but that's the struggle you might put up with. Which some people would prefer to pay gold or cash to skip. - Same reason "pay to win" micro transactions can make it in a lot of games.