Knowing of course, as a frequent flier, that cabin crew have no means of accepting payment onboard or reticketing.

Refusal was therefore perfectly reasonable, and not something to whine about here. One can only speculate about your motives for asking something you presumably knew would be refused, but power banks are cheap and readily available, so maybe worth getting one?

Otherwise there are plenty of feasible opportunities to pay for the class you want to be travelling in, either by booking it in the first place, UUA, POUG or AUG.