This is definitely part of it. The nature of these recent trades is definitely relative. I remember when Dan Haren was traded for Mark Mulder, and yet I haven't heard that come up once in all the teeth-gnashing about recent deals. I think it definitely has to do with a few things:
-- The struggles the Cardinals have had on offense.
-- The selective/recent performance of the traded players.
-- The build-up of runnerup finishes, true or perceived, for some players.
-- The departure of Heyward that shook the brand a bit.
-- And, yes, the fact that people still point to how the Cardinals traded players who never latched onto the majors for Holliday, Walker, and others -- and all that without acknowleding the game has changed since then, truly.
I also think it's entirely natural for people to dream on players, and that's the Baseball Paradox. It's a game of failure, and the more someone plays it the more failure there is, and the less room there is to dream about the production and more evidence there is that the player will fall short of that production. It undercuts some really good players who are consistent, are solid, and just aren't the stars people dreamed them to be.
For a long time the Cardinals made deals with players going out that never caught on, or did in minimal roles. Alcantara, Arozarena, Voit, Gonzales, Gallen -- they are all having success, and thus that success coupled with the Cardinals' search for offense stands out relative to the trades long forgotten. It's part recent. It's a large part relative. And some of the misses are real.