One downside is that seniors who pour their blood, sweat and tears into their college careers wouldn't get a bowl game to cap that off. That would really suck.
Thing is, to your UGA call out, every year there are big time schools who's non-playoff bowl appearance is a let down. There are also schools like ours where an unexpectedly good year and an appearance in a Camping World Bowl can draw 25K.
I firmly believe that a 13th regular season game will be added this decade, but it won't come at the expense of bowl games.