There are enough current M3 owners that will be selling their current car and getting into the new M3/4 that supply should go up pretty sharply in the next couple years. Assuming demand stays constant, that should mean prices will be depressed some.
That said, I honestly think a lot of it depends on the initial reviews of the new car. It will surely outperform the current car in every measurable statistic, but if enough influential car journalists feel like something has been "lost" in the next iteration, the perception may arise that the E9x is a true "end of an era" car, and that may motivate more current owners to hang onto theirs and others to obtain one (this is playing out now with the old/new GT3).
Mine's a keeper.