So having sat down and mapped this out, this is my opinion on where things could end up -
SHORT TERM (0-5 years)
ScotRail
As has been reported in Modern Railways, there is currently a cascade plan in line with electrification in Scotland. Therefore, if this is followed, I do not anticipate much if anything coming south of the border unless absolutely required and all Sprinters will go to scrap once taken out of service.
TfW
Out:
36 x 150/2s > Northern
24 x 158s > GWR
11 x 175/0s > EMR
16 x 175/1s > EMR
It would make sense for the 158s to go to GWR to replace the 12 Castle Sets given they already run on part of the route. The 27 x 175s would directly replace the 26 x 158s at EMR for the LIV-NRW route and give not only a capacity increase as trains could run as 5-cars but also a train which has recently be refurbished (something EMR desperately need). 150/2s to Northern to displace the 150/0s & 150/1s to GWR and create a homogenous fleet. 170s have already been accounted for going to EMR and 153s will hang on until someone decides to put their hand in their pockets for West Wales/HoW.
EMR
Out:
26 x 158s > Northern
20 x 156s > Northern
In:
TfW > 11 x 175/0s
TfW > 16 x 175/1s
158s to Northern to displace 150/0s & 158/7s to GWR and provide additional units, 156s to Northern to replace 155s and provide additional units. 170s from WMT/TfW/Southern already accounted for. The 27 x 175s would directly replace the 26 x 158s at EMR for the LIV-NRW route.
GWR
Out:
12 x HST Castle Sets > scrap
20 x 150/2s > Northern
In:
Northern > 6 x 150/0s
Northern > 46 x 150/1s (not all of them, rest sent for scrap/spares)
Northern > 8 x 158/7s
TfW > 24 x 158s
158s from TfW to displace 12 Castle Sets. 150/2s to Northern to create a homogeneous fleet. All 150/0s and some 150/1s from Northern in return to run Devon & Cornwall branch services & provide capacity increase. 158/7s from Northern to provide additional capacity for GWR regional services.
Northern
Out:
6 x 150/0s > GWR
46 x 150/1s > GWR (not all of them, rest sent for scrap/spares)
7 x 155s > scrap
8 x 158/7s > GWR
In:
TfW > 36 x 150/2s
GWR > 20 x 150/2s
EMR > 20 x 156s
EMR > 26 x 158s
All 150/0s and some 150/1s from Northern to run Devon & Cornwall branch services & provide capacity increase. 155s to scrap when they are displaced by 156s from EMR. 158/7s to GWR to provide capacity boost, replaced by 26 x 158s from EMR. All 150/2s end up at Northern to create a homogeneous fleet, 156s and 158s from EMR to replace 150/0s, 155s & 158/7s plus provide additional rolling stock.
So realistically the only Sprinters that could potentially go are the 150s (by my prediction the 150/1s) and 155s plus the Castle Sets.
MEDIUM TERM (5-15 YEARS)
The most pressing things to replace will be the 150s. GWR ones are likely to be replaced with something similar to Class 230 battery power if successful on the Greenford branch. That leaves about 82 150/2s at Northern to be replaced. Depending on further electrification schemes it will likely be battery/AC metro trains with some internal cascading. Latest 150s will go will be 2030.
The 71 x 156s, 129 x 158s and 30 159s will likely be replaced with bi or tri mode trains dependent on electrification and advancement of alternative fuels. 156s likely to be around until 2030 and 158s & 159s likely to be around until 2035.