Instructors will still need to hold the license and rating for which they instruct, so there will still be some demand for instructors holding those ratings.
But for every 100 pilots graduating with a single pilot CPL/IR today, how many will only ever be doing single pilot work? I guess only a handful. In 10 years time if the MPL takes off, I think the vast majority will have MPL licences.
In that case supply for air taxi / single pilot ops will dry up, costs increase, and eventually it will be cheaper to employ 2 hour-hungry MPL pilots!
Perhaps single pilot operations can be a rating added to your MPL, after sufficient further advanced training. Who knows?