Correlation does not mean causation.
Vanilla and TBC had growth because more new people were joining than old people quitting.
WotLK plateud because either A) the rate of new people joining and old people leaving were roughly equal, or B) No new people were joining and the old players were happy with the state of the game
Cataclysm declined because more old people were quitting than new people joining
Did some quit because of the cash shop? I wouldn’t doubt it, but you can’t say definitively “muh cash shop killed the game”. Their reasons for doing so are pure speculation.
I personally got burn out mid Wrath and quit, came back to try Cataclysm and didn’t really have something to keep me playing so quit again. Didn’t feel the need to try MoP or WoD so didn’t try WoW again until Legion. The cash shop had nothing to do with my decisions then.