You are assuming any title win necessitates being a "Vince guy." We clearly have established throughout history that it is not always the case.
When it's BROCK LESNAR, you're god damn right that any title win necessitates being a Vince guy. Vince McMahon is not putting a non Vince guy over BROCK LESNAR. For ANY reason.
Basically what this equates to is you essentially saying that because Vince McMahon gave Carmella the womens title once upon a time, he's going to allow her to beat Ronda Rousey. THAT'S how steep the difference is between Brock and Rey.
Remember, a number of guys over the years have held the Universal title without Vince ever really being that high on them (Finn, KO).
And was the champion Brock? No. And I think Vince was high on Balor, but he lost faith in him when he got injured. KO was the result of an injury and a lack of options. None of that means anything because I'm not debating the idea that Rey can't be champion. Of course Rey can be the champion. Rey can't be the champion RIGHT NOW, because BROCK is the champion. Difference.
Vince may obviously never give guys like Joe or Nak a top title, but there are a certain bunch of names you can see he would, because he HAS before. Mysterio is right on the cusp of that list. We've seen Vince give Mysterio titles and then book him like a joke, we've seen him give Mysterio a title and yank it off him after a few hours...but the point is, he DID win them.
And that's my point. Vince is not going to put a guy over Brock that he treats as a joke.
These are the people who have wins over Brock Lesnar since 2012, since he came back.
7 years, those are the ONLY people who have beaten him. What do they all have in common? Golden boy, golden boy, golden boy, golden boy, golden boy, golden boy.
Brock has plowed through AJ Styles, he's plowed through Daniel Bryan, he's plowed through CM Punk, he's plowed through Finn Balor, he's plowed through Randy Orton, he's plowed through Strowman, etc. Rey is NOT beating Brock. You're being worked. Brock Lesnar is not Randy Orton, Brock Lesnar is not Seth Rollins, Brock Lesnar is not Roman Reigns, Brock Lesnar is not even John Cena, Brock Lesnar is THE MOST PROTECTED WRESTLER IN HISTORY. Period, point blank. Those guys might find themselves in a situation where they might lose the title on a whim to a Rey Mysterio, or even a Jinder Mahal is Vince got fucked up in the head enough, as we've seen. Not with Brock.