Impartial Jets fan here...
I don't think anyone can deny that Alonso was a really good LB in 2013. Top 3 in the league in tackles if I remember correctly, plus he had four INTs on the year. Sure he tore up his knee and didn't play at all last season, but this guy has proven he can be an elite play-maker on defense in just one year in the league. On the other hand, you have a RB with declining stats (I know Eagles fans, the O-line had injuries last season) going into his 7th year and about to hit 1,500 carries for his career and has over 1,750 touches out of the backfield in total. It's just a fact that RBs have a short shelf life in the NFL and McCoy looked worn down in the last four games of the season when the Eagles finally had a healthy O-line in tact (believe me, I was well aware of this as a fantasy owner). While LeSean McCoy peaked in 2013 statistically, Alonso was having an excellent rookie campaign. Eagles fans should be happy with this trade, IMO. RBs are replaceable, while top-level LBs are harder to come by. As a Jets fan, I won't be happy going against McCoy for the next year or two, but after that, what are the odds he remains an elite RB? I'd say the odds are higher that he's backing up some young guy on another team or out of the league altogether. With the advancements in ACL surgery over the last decade, Alonso should be able to get back to playing at a high level for years to come, especially with an entire season+off-season to rehabilitate.