This fails to consider that different days off are considered as having different value to folk working in the industry .This for me is the essential issue with changing from Sundays as additional hours, ie overtime, to within the working week. The latter of course would give staff more time off, and the operator a more reliable Sunday service, but let's talk about money; Would those staff currently working Sundays as overtime shifts be happy to lose take home pay in return for fewer hours at work, or would the Unions demand an increase in basic pay to compensate ?
At least among the majority I know weekend rest days are considered far more valuable than mid week ones. Hence why there's lower appetite for overtime on a Saturday which in some places can cause shortages of traincrew . Especially during summer when everyone is on holiday and daily leave is maxed out. Sure I'll volunteer for a Monday or a Wednesday but not a Saturday.
Hence compensating traincrew required to work Sundays with a load of monday-friday rest days might not necessarily be accepted without other trade offs by the company .