The sprinter fleet is knackered and worn out but so is the 170 fleet which shouldn't be the case but they have been worked far harder I think than they were ever meant to be
The 170s aren't particularly powerful and struggle with the Scottish hills. I wouldn't be suprised if being run much harder is affecting them more than other turbostar operators.
Alternatively, the 222s are going to be much quicker to accelerate than the 170s so Scotrail will need to get them of Inter7City services if they want to speed up the timetable.
Or they just want the 170s elsewhere for short term sprinter replacement. The 170s have the great benefit of being able to work with Scotrail's remaining 156s, 158s, or the cycling converted 153s.