The magic of the prepay card was that nothing but discretionary spending ever went on it. When you combine that with the automatic category assignment it gave people this enlightening window into their spending that they’d never had before. And it was enforced “envelope” spending because it was a totally different account and so you could assign an amount you were happy spending at the beginning of the month and you’d quickly see if you were overspending and could slow down. This is also why the pulse graph was originally awesome, and later useless.
TL;DR: Please Monzo show us discretionary vs mandatory spending in Trends!