FFXIV manages just fine without public testing, even with a decade of technical debt and spaghetti code now. I can't even recall when I last actually encountered a bug in live, save the Raubahn Extreme bottleneck back on Stormblood launch.
The only reason I see WoW benefitting from public testing is tuning and finding bugs in how abilities/talents/trinkets interact, as WoW's design in that regard is extremely chaotic as opposed to a very sanitized, homogenized FFXIV.
In the end though you're right in it not being about public testing being there or not: you still need to invest resources and time in QA and professional testing, preferably in-house.
Blaming external QA is always easy even though the problems always start in design and development.