he's also older and about to become extremely expensive.
there's your trade off. depends how they see the cap evolving. i know people dont believe in the cap, but there does come a time when teams have to pay the piper.
how many huge contracts can the lions have? didnt we have this issue when we had stafford, suh, and calvin on massive deals? if we trade for simmons and he signs a huge extension, we'll have goff, simmons, sewell, decker, ragnow, st brown, hutch...how many more big deals can they take?
otoh, if you think youre one player away and he fits the scheme to a "t", maybe you take that chance that you lose one of the above guys?