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Honest non-shitposting answer... it's not really NXT fault, but the fact that there literally is no more talent in wrestling at all.
And that's not shitting on the current wrestlers because it's not really their fault either. It's just the pipeline of good enough people who are athletic and charismatic enough doesn't exist not is there any way for them to develop outside the indy system because A. the territories died i.e. ACTUAL WRESTLING COMPANIES not modern indy stuff which is a derivative and B. all the athletic and charismatic people go into social media, MMA and boxing instead because it's a far more viable way to make a living.
Lesnar/Cena/Batista/Orton started in early 00s when wrestling was hot and so they're pretty much the last generation of special guys who are big enough to make it.
You could argue Roman Reigns is up there, but firstly he wasn't even NXT he was FCW originally, he's like a second generation guy, and the reason Vince picked him was because quite simply who the fuck else was there? Depending who you ask here it's debatable if he was even a legitimate draw.
TLDR - It's not that NXT is a failure, it's just that wrestling died two decades ago so it's impossible to recruit potential megastars who are in other industries now.