To throw in my view on the Welsh situation, we've only got 30 pacers, compared to Northern's 100 or so, which means our problem is of a different scale.
We're expecting 12 170s and a gaggle of 153s in this year, as well as 5 230s and (hopefully) some nice, long 769 units that will each replace a double Pacer (originally 4, now 9 on order).
On that evidence, there shouldn't be any issue with withdrawing all Welsh Pacers at the end of the year. The trouble is getting all the Sprinter fleets upgraded to PRM specs before 2020 as well.