Casters had almost no mobility in classic/BC and it was fine. It will be fine again.
However I'm concerned about the inconsistence I see in this regard. For example disc priests have practically nothing to cast on the move (except for PWS), while holy priests have multiple things: pws, renew, coh, holy words. Most trash pulls can be healed while running non stop as hpriest. Trees also still seem to be pretty mobile.