Honestly, I wonder if the universal DH has something to do with that.
Back in the day, if you had two good players at a position, it was time to cut bait on one. The Phillies had Jim Thome and Ryan Howard, and no way to play both of them. So it was time for Thome to go. Now you'd see them both splitting time in the field and at DH.
The universal adoption of the DH and the popular method of rotating players at that spot rather than having a set DH may be a significant reason why we don't see so many of those trades anymore.