True, Shields is not a clear upgrade on Stroman. IMO, there is still some uncertainty over Stroman, sophomore jinx and all, whereas Shields is close to a lock for 230 IP and 4 WAR.
A starting 5 of Shields, MB, Dickey, Stroman, and Happ is damn strong in a contending year. And Hutch returns at least an average 2B in trade (Cubbies).
Then after MB leaves, you still have Shields and Dickey to lead the youngsters, Stroman, Norris, Sanchez in 2016. As it stands right now, in 2016 there will be Dickey, Happ, and the aforementioned youngsters.
Shields makes a lot of sense now and going forward.