He plays a premium position, that is very valuable. Reyes wouldn't hit enough to be a good corner outfielder, given that he's played infield all his life he would probably not be Alex Gordon out there.
It was a good play obviously, but it's not like a handful of second basemen couldn't have pulled it off. The slow turf probably helped slow down the ball enough so he had time to get there.
Wade Davis was not a good starter. Were he to be 2010 or 2011 Wade Davis as a starter and he would not help this team out much more than a Marco Estrada.
I don't think his decline will come that soon and won't be too bad, Miggy is a special hitter. On the other hand you also acquired an underrated ace before he fully breaks out. This is an excellent deal for you.