It can be done if all existing drivers are offered the chance to bring Sundays inside, with all new joiners coming in on that basis, but with those who wish to keep their Sundays outside being put into a separate link.
But the drivers (orat least their reps) at some TOCs will not accept that. Personally i don’t understand why, but then there’s lots I don’t understand.
Actually it is more a case of putting the cost into different buckets, and DfT only counting one.
I have yet to see any evidence that employing an extra driver costs more than say a driver not working 2 services on a Sunday and there being 200 delay repay refunds.
It is as if someone seems to know cost of the extra driver, but completely forgets the cost of the cancellation (and amount of fare likely to be refunded) due to lack of driver.
Oh, they know.