They didn't change stuff in
WotLK (system mostly stayed the same, content organization/realization mostly failed), just tried some stuff, for which were get some strikes in head from community.
Cataclysm was transition (legalisation of changes, same as WotLK there was MoP+WoD "try" period for Legion) part. As for your words about
Legion... any expansion is result of work and decisions of devs, don't you think that your criterion in such context completely doesn't fit in role of comparison parameter, eh?
I'd strongly criticize your "
what they did in Legion was working"&"
much needed change" part (one of most incorrect and unsuccessful decisions in all areas, from social to organization of content and gameplay in general, were made here - only decisions made during Cataclysm's implementation can compete with them; and no, they didn't benefit system, people who criticized Legion's ones even before it began were completely right in predictions), but topic isn't about that, so *shrugs* this is your opinion, I heard and took note of it, thanks for sharing, but, sorry, it isn't change anything for me.
ps. There is no point in talking about success in terms of
popularity only in terms of efficiency, adequacy and viability of system as a whole (for the same reason, I already mentioned this in topic about
best expansion, there is no the best! one).