Joe isn't a "rookie". Joe can be shaped into whatever they want to shape him to be.
Nobody whose name has been thrown around is a "rookie". AJ was a WWE "rookie" and look how quickly they got behind him. They can frame these guys as "rookies" and do away with their past, or they can embrace their history and get people to buy in. You can sell the fact that Joe has been around the world destroying people before he got there. They tried to bury Danielson's past, it didn't work.
Joe isn't some green newbie, he isn't Roman Reigns, he's a well traveled veteran who has been around the tracks a long time. Ultimately WWE can try to package him as whatever they want to, but it makes it even easier when it's actually true. The guy had wars with CM Punk, he's fought with Seth Rollins, Sting, Christian, Steiner, Bryan, AJ, Angle, Jarrett, Raven, Booker T, the Dudleyz, Nash that's just some of the names of guys who have been in WWE that he has wrestled with. Huge names. WWE can acknowledge that, and frame Joe as a guy who has been up against hall of famers and multiple world champions and beat them, or he can be a "rookie".
But even if they were "rookies" they don't actually need to be someone with "established credibility" because beating Brock gives someone instant huge credibility. If Joe beat Lesnar, people aren't going to turn around and be like oh but Joe isn't credible, because they just showed that actually he is. If you put him in a savage war with Lesnar, that's how he becomes "credible".
The idea that anyone to beat Lesnar needs to actually be seen as a credible threat to him is flawed. In sport those events are called "upsets" you can bring a guy in who "nobody gives a chance", and then he wins. Suddenly perceptions of that guy change, because he's the guy who beat the beast.
Orton's career makes him about as credible as one can be aside from Cena. Realistically nobody is a "credible threat" to Lesnar. If someone beats him it will be an upset.
Still yet to hear a reason why WWE should be slamming Brock hard. In fact your approach only reinforces reasons why it makes no sense to do so.