I mean, technically “retail” has started with EVERY current expansion.
But I see what you’re trying to do, you’re trying to establish roots, which is absolutely fair game. But by THAT standard, WotLK would be the origin point.
Much of what drives modern design principles in retail IS WotLK designs that have naturally been iterated on!
Cata, ironically, was the expansion that TRIED to curb things back towards some TBC/Vanilla era traits BUT those were the things the majority of players hated. Harder raids and bigger jumps from lower to higher content was perhaps THE most “classic” experience we could go back to and THAT marked the peak and decline of WoW, outliers removed.
While people like to point to MoP or WoD for such roots, those people also miss the systems and designs in place as of WotLK when looking through their rose glasses!
The reality is that “retail” is ever really a snapshot of “current” WoW, but the “middle” and “modern” WoW eras are what you’re lumping here.
If we grouped expacs based on count, then we could lump Cata in with “Classic” and start the mid era with MoP BUT that still means the early WoW era made up LESS of “what WoW has been” than the majority of expacs.
Cata is also an outlier among the first three end games, as it was more iterative and loose than WoTLK which was already the grandfather of retail; Cata probably shouldn’t even be lumped with the Classic Era of WoW, and more accurately marks the middle era’s start in terms of design AND systems iteration!
If we go with JUST design? That’s WotLK.
If we go with JUST systems? Then MoP had the bigger jump BUT because of the design, which started in WotLK.
Either way you slice it, Cata was the biggest departure from a given era of WoW any given expac has seen, until Shadowlands. With the exception of SL, basically EVERY expac since Cata had smaller bits of iterative designs specifically to address problems of the expac before it.
Much of those problems also date back to…WotLK!
WotLK addressed Vanilla and TBC design issues but sprung up others as it did. And that’s just the nature of iterative designs in a seasonal model of play.
Thus, the point at which the “retail” mindset of iteration begins is the point at which it was first incurred; WotLK