I don't think it's a no-brainer at all. If Cousins has the promise of a franchise QB (and with a headstart on NFL experience compared to rookies to become one), you're trading that away on the assumption that Griffin is going to work out his kinks and become a great QB. That's far from a given and I doubt the coach's concerns will be answered by the free agency period and before the draft when players are frequently traded.
If you do trade Cousins this offseason and roll with Griffin, the pick(s) from Cousins will be nice (and likely needed), but a franchise QB is the biggest piece to a puzzle in this league. So if you trade Cousins who ends up a franchise QB or who might be one, and Griffin completely drops the ball, I don't think the pick(s) will replace the hurt of having traded the better pro QB and having to rebuild from the ground up once again. Because honestly, if Griffin doesn't have it going through next season it's lost.
Edit: Forgot to add why I don't think it's not a no-brainer. If you roll with Griffin expecting him to develop into a franchise QB, and doesn't become one, but Cousins could be one, you have a headstart on most bottom-feeding franchises without a QB.
Of course you could roll with Griffin and both could blow it, or Griffin could blow it and you lose out on the pick Cousins might have brought, there's a ton of permutations that could happen. I just think our bets are best hedged with Griffin with a long offseason, and Cousins playing, learning, and maybe competing for the starting gig in the summer.