From what I remember that was more "I've got your back, bro" than it was "I'm coming for you, motherfucker"
To be fair, him stopping Rollins from cashing in at Wrestlemania would be precisely the same thing since it would be his former Shield brother Reigns that Seth was cashing in on. Plus, despite the fact that being distracted and drawn into a feud with Wyatt cooled off a lot of the vitriol hatred he had with Seth at the time, he still thinks he's a scumbag and, were he to actually
cost him the briefcase, would still get a lot of cathartic revenge out of it for what Seth did to him (and Reigns) the year before.
I think it'd be a waste of the briefcase personally since you could rekindle the Ambrose/Rollins in a number of other ways, but I see the story logic behind it.