Plus they can just bring him back lickety split if things aren't working out.
And it's not a big deal given that they're not losing the voice actor from it.
The plan may be to bring him back anyway, and this was just to create some publicity and ratings. Despite them making a point of getting rid of the time machine, it still stands to reason that Stewie is the most likely person to bring someone back from the dead, and the character he'd be most likely to go through that for is Brian.
Or maybe it's just wishful thinking after the disappointment of his replacement. He just doesn't feel like a well thought out full time character. He seems like any other "character of the week" type deal, good for a few stereotype jokes and that's it.