"Infill" is always the hardest projects to get off the ground, there are many unelectrified chords and short stretches that have been dropped from scopes in other projects, including GOBLIN and GWML. If the MML becomes just another infill with perfectly functional services on it, we'll be looking at the same conundrum when it comes to replacing the AT300s.
I was thinking about this recently. Once you have paid more to buy bi-mode hybrid rolling stock the argument and case to complete the electrification becomes more difficult.
Cascading the bi-mode stock and replacement with full electric would be one option but rolling stock cascading isnt political great as Northen MPs are finding.