I had not thought of it that way - but yes. Also no pressure from the Persons of Reduced Mobility legislation deadline because all trains in future will be compliant anyway.And also far easier to push for sign off on the last 25% later as well.
Some bits will be technically harder than others needing lots more design and optioneering
Some bits will be more disruptive than other
Some bits will be much quicker to complete than others.
Hence there would probably be a 4-5 year range in completion dates between the first and last section even if all were approved now. Hence the Bi-modes allow each section to be used as it becomes available.
Unlike many previous scheme there is less pressure from electric only new stock being ordered and delivered. Flexing delivery date not cost to hit a deadline will be the name of the game.