Not going into details as I'm still not sure who's lost what, I guess VT are hoping to reduce refresher days and eventually depot compliments.
Depot compliments are the agreed number of traincrew at each depot, TOCs often want less than the ideal number & cover work with overtime, again it's down to money.
I suppose it'll not have made much financial sense to keep all traincrew refreshed on a route only used for a maximum of 2 or 3 weekends a year for diversions, maybe a new franchise could include a regular or summer only serviceWith the volume of traffic on our roads and the readily available paths on the S&C plus the millions spent to reopen the line I am amazed more pressure is not put on TOC's to use it!
How do you do that? If they don't want to run the trains you cannot really stop them unless it is a franchise breach.
There is no incentive, they get the costs of the buses covered, thats it. They actually get more by diverting as they are compensated for journey time and mileage. Make it worse for them and they will refuse extended engineering blocks entirely.