Shortening consists to match the lower demand is one thing - given the fact that most Monday and Friday commuters are likely to be 5-day-a-week commuters, cutting service frequency on those two days may well lose the railways their custom (and money) on the other three days as well. If cutting frequencies means a journey prolonged by 5 or 10 minutes that's probably tolerable - any more than that and people who have alternatives may well start to use them (especially if high cancellation rates continue). If you're talking about adding extras Tue - Thu that's different - but don't forget Saturdays as well, although the extras will be needed at different times and in different places.