I anticipate that the requirement to pass all of the CPL writtens in order to commence the FI course will keep the number of PPL instructors down; frankly the numbers are likely to be too small to impact upon current FI jobs much.
As for the suggested primacy of the MPL, this assumes that the only jobs in flying are with the airlines, and that there are no air-taxi, instructional, or just plain single-pilot jobs, whereas the truth is that there are lots of such jobs. Also it assumes that the majority of airlines will sponsor all their new direct entry F/Os. None of those things seem likely to be true now or any time in the foreseeable future.
G