Was it even a SWT decision?
I suspect
RG decided that they might make more money by using trolleys and at the same time reduce costs by not having to maintain the buffets (cleaning, stock rotation and general maintenance etc.)
Either way I rarely want anything when the trolley passes by but on occasion do when its elsewhere, mind you RG prices are the main reason I try and buy a coffee etc. before boarding.
Don't think I have bought on board on a SWT service for some years now, however I have taken great delight in depriving RG of a free SWT paper cup and its contents of flavoured hot water on occasions.