I didn't think the QB Transition tag is that high. Is it? Transition tags are meant to encourage negotiations toward player movement, but with an out for the Team.
Then offer Mallett $6-10 million and let another team match or beat it as a counteroffer. The object of the exercise is to keep him if economically feasible, or obtain a good compensation, if not.
The point I was making is that one year of grooming for an FCS level QB may not be enough, before he is a ready backup QB.
As to why Ryan might accept, I think he might prefer the position of Aaron Rodgers versus say Blaine Gabbert or the dozen, misused QB candidates of the rechartered Browns.