I think that there would be a serious political stink if services were allowed to be reduced due to a lack of strategy on bridging any gaps between 2020 and the arrival of "new" stock, even if the various ROSCO's and TOC's have to be compelled to keep the 142's going until replacements are forthcoming.
Hmm, time to speculate again on where the cascaded replacements might come from, lets see: -
1. North west Electrification - how many Diesel units directly replaced
15150?
2. Crossrail/GW electrification, indirectly @ 45 165/166 cascade aprox
8 143 &
37 150 Diesel units.
3. Scotrail EGIP, indirectly 170s cascade @
10 156s +
2 for Cumbernauld. More if Dunblane/Alloa electrified.
4. Bedford - Bletchley, together with Walsall maybe
3 diesel units(probably the remainingg 3 150s).
5. South Wales valleys electrification - 15 142s
replaced directly,
15 143s &
10 150s cascaded.
Roughly then thats 23 143, 55 150, 10 156, total
88 to cascade to Northern to replace 79 142, even allows a bit for growth!
My figures are estimates erring on the cautious side, I am not familiar with the north Western route requirements, so if you have more detailed info feel free to correct me.
And yes it does require the Electrification/Cascades/Gauge clearing work to complete well in time for 01/01/2020!!