This is very true and surprising that it wasn't factored in when they were taken on. They are planned to stay long term too and work down West & HOW.
The initial plan for 170s, 230s and 769s may have seemed a bright idea at the time, and possibly the only idea at the time, but certainly hasnt gone to plan.
Give Northern all the 170s in exchange for 150s or even 158s until the new trains start arriving!
There's 12 170s and probably only about 6-7 in use at most on one day (pre-covid). At least with a more uniform fleet of 150s or 158s there would be less out for maintenance and no more crew training is required, with better usage to cover 769s.