With hindsight, it seems that season 12 was when Seth MacFarlane told Fox that they needed to give him complete creative control to do whatever he wanted.
The result was an episode that fails as both comedy, and drama, because there was never any reason doubt that MacFarlane writing out what he'd already told us was his favourite character in favour of another voice actor would only ever be a temporary stunt.
That 25,000 people fell for it and demanded the inevitable return of Brian was rather sad.
The episode isn't a total dud: there are the usual racist tropes, and a couple of moderately well observed character jokes even after The Incident.
But the core concept is rushed and forced, like the rape jokes that MacFarlane is obsessed with making.
At this point we were already four seasons beyond MacFarlane's interest with the whole show, and he was already busy stinking up the screen elsewhere, in Ted, and hosting the Oscars. Everything since then has just been managed decline.