This is one of those also/and situations and not an either/or.
The audience is fickle af *and* face Seth has gotten a bit boring. Both those things can be true at the same time. Let's keep it 100, WWE audiences are notorious for turning swiftly and without mercy and sometimes without any real reason (ex. Ronda Rousey, Bayley) so it's a valid thing for Seth to call out. On the other hand, he's said some things that have rubbed some folks the wrong way and the audience fatigue w/him was evident way back during the Ziggler feud so this is hardly out of the blue. And as far as Brock goes, let's just call a thing a thing: A lot of us would have gotten behind a random member from No Way Jose's conga line if it got the title off Brock.