A large amount of WWE's biggest stars the last 25 years are people who barely or in some cases never worked outside their system and could be molded practically from scratch. Rock. Cena. Roman. Brock. Angle. Batista. Orton. Trish. Bianca. Usos. Charlotte. Sasha. Bellas. Kofi. Big E. Wyatt. Lashley. Alexa. Miz. Ronda.
Bron/Tiffany/Creeds are probably the biggest male/female/tag prospects in NXT right now and none of them really worked indies either.
If someone really travels the world before making it to WWE they're probably going to be in like their mid-late 30s by the time they arrive. You can still get an AJ Styles strong run but your absolute face of the company type people are probably going to be the ones you get early and can plan to get a decade plus out of.
Rock came from a wrestling family and got his start in USWA learning the ropes.
Cena: You could argue OVW which was when he debuted, had only recently become a farm system for them
Angle: Olympic Gold Medalist. He has innate ability
Orton: Again OVW which again was training people in the typical indy fashion
Charlotte: Wrestling Family. But sure I suppose
Batista: OVW
Wyatt: Wrestling family
Lashley: Arguably only became really good after leaving WWE
Ronda: MMA pro, again innate ability
Kofi: Worked the indy circuit before WWE
Miz: UPW before WWE
Roman is a fair example.
Their new belt was put on Seth; Former ROH guy
Tag Champs: Kevin and Sami: Former ROH guys
IC Champ: Walter: Years in the indy scene
NXT NA: Wes Lee: Former TNA guy
NXT Tag CHamp: Gallus: Indys before WWE
And If I have to name everyone they use now that had indy experience/exposure: Styles, Balor, Anderson/Gallows, Pete Dunn, Drew, Viking Raiders, Damien Preist, Gargano, Gacy, Kross, LA Knight, Riddle, Ricochet, Roode, Sheamus, Nakamura, Rey Mysterio, Ciampa, Woods.
I get what you're saying but very few of the big names they have were from scratch. Did they learn the WWE style? Sure absolutely, but wrestling companies have always relied on getting big names outside their own system.
I would say only using new talent would be bad but seeing as how the E is currently in their 'everything works' era they'll likely pull in a 3 mil rating even if they put on two green as goose shit rookies in a main event.