I wonder if this has been a sudden idea of how to fix a problem (temporary fix?) or if the idea of dropping to a Sunday service has been a back burner contingency idea for some time?

Long term (depending on how long these emergency measures last) , its only going to confuse passengers and force them to use other transport options such as a car or a rival operator (or Metro). Whilst Covid has devastated many industries, I genuinely don't see how public transport as a whole can get the publics confidence back to where it was prepandemic (and even then public transport had image issues and stigmas attached).