I thought it was the pros, yep, but wanted to know the full answer before giving my opinion :)
I don't know if Line6 is "powering" many bands, but for instance I saw Smashing Pumpkins using Helix Racks a few years ago (although mostly for effects, because Billy won't allow digital amps on stage xD).
On the other hand, I'd say arena bands usually have a tech guy who makes the programming for them, unless the musicians are nerdy enough. And if they are, I'm sure they would appreciate a way friendlier UI.
So again, I don't see why the Stadium wouldn't be embraced by that market if emulation quality ended up being identical. And there's also profiling, which Fractal still doesn't have.
The only remaining reason seems to be "not having to learn another interface", but how long will that happen? They will eventually have to learn a new interface when a new Fractal device is released (at least I hope they don't do like Kemper and take UX very seriously, for their own good), and Stadium's seems quite joyful to learn, kind of like when you switch from Pro Tools to Studio One xD