The trend has been down since Vincent Jackson's deal. Even overpaying Mike Wallace got him nowhere near the previous top deals (Fitzgerald, etc.).
That plus very good rookie contributions (no more waiting 3 years) and a decent draft class seem to point to a depressed market. It has not fallen like RB yet and so predicting it is a dicey business. But it is quite stagnant.
You are right that the one group doing better are the slot guys. Like I said, with an increased cap coming, there is no guarantee his market will be flat.