Here's the problem... Brock already had an attitude of "get in get paid get out". He says as much in his book, he didn't want to be a pro-wrestler as such, he just wanted to make money in a short space of time. He also had a problem, rightly or wrongly with the schedule he was being asked to handle. Others did and continue to do so without complaint but had this not reared it's head when it did, it would have soon after without question.
Factor into the mix that as soon as Brock's big push was in motion, he was also seeing guys like Nash and Goldberg parachuting in on even better deals than he was on, bitching and moaning and politicking. So that too would have become an issue for him. He'd also seen Austin walk out while the "star of the show" and that it could be done... he'd seen Rock go to his initial Hollywood schedule of only being onscreen every 3-4 months, so he knew he didn't HAVE to be on TV all the time.
Vince had worked out a deal with Brock, even offered the use of his jet for a spell till Brock decided to get his own. Vince wanted Brock to be his "face of the company" that badly he was willing to concede a tonne of ground, but not control.
But Brock didn't want that... he wanted it all on his terms and he saw UFC as his meal ticket, he went through the NFL tryouts almost as a ruse but that Vince gave him the no compete and initially Brock signed it kinda shows the mentalities involved... Vince is saying "I want you here with us...if you're playing football, that's fine but you're not wrestling or fighting...I'm not letting you go for that..."
Brock wanted to do just that of course, that it kinda fizzled out in court was perhaps more to do with Vince realising he didn't NEED Brock rather than him not wanting his no compete clauses tested in court, as there would have been more than enough evidence that Brock had said he wanted to play football, not fight UFC to at best make Brock seem duplicitous. Vince had some luck... JBL worked out far better than possible - Eddie had gotten more over than expected and Cena, Orton and Batista had all gotten to the required level quicker than he dared dream of... He still had Shawn, still had Kurt, Taker and Kane - even Show... sure losing Brock was a loss - but not a fatal one, especially once the story of how he'd "faked out" with the NFL stuff in favor of UFC got out... Brock would (and did for a while) appear the dick in the situation, especially when he lost that first fight... but by the time he beat Couture people had forgotten that and so had Vince as he had shiny new stars like Cena.
So for the sake of argument, Brock isn't leaving at WMXX... He beats Goldberg quickly as he definitely is leaving and under a cloud. The ending of the show goes as it did with Benoit and Eddie in the ring when Brock stomps to the ring and stares down both men...
It sets up a period of time where Brock stalks both guys, of course people think he's going after Eddie because he lost the title to him but it's actually Benoit who he ends up facing at Summerslam.
The problems start when Vince want's Brock to lose due to Eddie's interference/saving Benoit, setting Brock up for a shot at Eddie's WWF title.
JBL doesn't happen in this scenario in the way he did, he goes heel and feuds with Eddie in the interim, but doesn't win the title when Brock invades the match in retaliation.
JBL ends up with the US title instead, Cena perhaps faces him rather than Big Show - either way he doesn't get the same rub that can see him win the World title a year later.
Randy Orton doesn't become what he does, nor Batista as their window is ruined - Brock would get that title back and he wouldn't want to lose to either of those guys or would walk before he did - thus devaluing them... Cena, forget about it.
Eddie and Benoit, they'd get their feud apiece out of it before Brock's politicking would see them downgraded. they might still play a big part in the shows, but the damage is done - I doubt Eddie survives any longer than he did as the damage was long done to him, indeed I could see a match with a pissed off Brock hastening the heart attack if anything. Benoit becomes bitter that his chance was taken away, perhaps even starts a shoot with Brock... Actually, I see Benoit coming out on top but that being the "cardinal sin" and it ruining his career and sending him to a dark place...
Angle and Shawn are almost the forgotten guys, Brock would avoid facing them where possible - Shawn cos of the HHH connection and that his new power with Vince would rival his own and Angle would go pretty much as he did or be the casualty instead of Eddie... Guys like Lashley, Rey, Umaga and even Edge struggle to get into a Mania main event till much later if at all and Punk probably never sets foot in the WWE.
Taker would be Brock's golem... he'd refuse to work him as long as possibe for that fear of "being fed to Taker", Taker would not be amused, tenstion would rise and one of them would eventually have to go... that's where Brock would come a cropper... Vince would not sell out Taker for him and come Mania 22 we're in a similar position, only Brock no-shows/pulls a Shawn injury rather than lose to Taker at Mania.
Either way - no real good comes from Brock staying in the WWE post WM20... Even now his value is questionable, on a cost to draw basis... At the time he left, I am sure it was a massive blow to Vince but looking back I think he'll know he dodged a very big bullet... If Warrior was a headache, Brock on the roster through the mid-late noughties would have been a migraine from hell... He'd never have gotten his "golden boy" in Cena, his PG era... Brock pounding people to a bloody pulp would not be PG and after a time, who would have stopped him?