The railway isn’t run by accountants and wasn’t in BR days. The decisions on locos and stock were/are largely taken by Commercial and Operating Management in conjunction with Engineering colleagues. Accountants are there to tell you where the business is heading, how much things cost and to compile a budget, based on the funding envelope the Business is either given or decides it can live with.
Loco Hauled Rolling Stock did not suddenly disappear at Privatisation. Most of it had gone before that date as the various businesses (& regions before them) decided that keeping spare stock for dated services was a waste of time, effort and money. Apart from the WC and XC Mk2/3 fleets and the day coaches for the Sleepers, very little got passed over to the ROSCOs and what was left wasn’t going to have a long life, even if BR had remained.
So, getting back to the 222 fleet, both Lumo and the Scottish Government will be leasing enough stock to provide 7 day a week ordinary timetabled services, not weekends extras to the seaside.