With the rumor being Brock stormed out of Raw after contract disputes with Vince, what are the chances Brock just says screw it and shoots on Reigns at Wrestlemania? We know Brock can be a hothead anyway, so what better way to tell Vince and the WWE to F off than take care of the "next big thing" on your way back to UFC? It's not like Reigns (or anybody else for that matter) could stop Brock if he decided to do that. That IMO would be a true wrestlemania moment. a) No one would understand what was going on.
2) Brock doesn't give enough fucks to do that. He's a smart enough businessman to know that it's not worth burning that bridge anyway. Brock cares about money and Brock and I really doubt he gives a shit about laying down for Reigns, even if he was leaving.