The DfT is not spending £78m on electrification and platform lengthening just to enable the 769s (which cannot operate in 6-car formations, so will be unable to make use of the platform extensions) to burn a few litres less diesel fuel on each working. £78m buys a lot of diesel!
Once the wires are up in 2024, I will be very surprised if the 769s continue to work Wigan/Southport services. Wigan - Bolton - Manchester - beyond will be 6-car EMUs and Southport - Wigan - Atherton - Manchester - beyond will be DMUs. And I am doubtful that Northern will redeploy the 769s to routes elsewhere on the Northern network, where they would be further from their Allerton maintenance base and would need to be worked by crews from different traincrew depots than those who currently sign them.
The 769 conversion programme was originally funded as a direct consequence of the postponement of the electrification of the Lostock - Wigan and Victoria - Stalybridge lines, originally promised for December 2017. Once those lines are finally wired, the Northern 769 fleet will have lost its raison d'etre.