Thing is even if you look at the complexity of these systems they are less-so than modern day WoW, a lot more is baseline. I.E: If you’re a Protection Paladin Avenger’s Shield is a given. You don’t have to work your way through to Shadowform on Spriest. This is why people are saying they have been baked into the class.
MoP and Cataclysm also use talents and glyphs and have some of the largest player number drop-offs.
Ergo, it isn’t glyphs that made players stay, nor was it glpyhs that made them leave. WoW changed, we changed and the MMO market grew bigger (at the time).