I think you are underestimating what Dahlin coming here means. Risto gets pushed to the 2nd pairing where he belongs, Tavares plays #1 Center, Eichel is #2, Mittle is #3 and you trade O'Reilly to Montreal for some other really good stuff (he becomes expendable). Add in Guhle, Reinhart and possibly a bounce-back year from Okposo?
If Tavares did come here, it would not be long before Buffalo is not only in the playoffs (next year), but probably contending for Cup by 2019. The vision is pretty easy to see if you add him to the mix.