Personally I think that this is the dominant reason that Monzo won’t expand categories; it’s about what the product will become later and not boxing themselves in. Though it’s very frustrating for us users who dearly love more fine-grained categorisation.
I was going to suggest just this, though you’ve beaten me to it by several months. It’s definitely the best trade-off if, indeed, preserving the general categories is important to Monzo. You could even have sub-sub categories. Why not let people define their own tree?
Yes, yes, double yes and another yes on top. At the bare minimum it shouldn’t be anywhere near the spending calculations - but as you say, it would be better to encourage financial responsibility in a more harmonious way. Coincidentally, I suggested inverting the role of pots the other day (albeit in an opt-in way) so that your salary could be assigned to a pot by default, and then your budgeted monthly outgoings would be transferred to your main account. This savings-by-default would, I think, encourage financial responsibility. I had a post in an old thread, similarly buried as yours, on the topic of “generate ideas for pots” (here, fwiw). It’s probably too complex / clunky, I concur: There’s something missing at the core around the way that savings are dealt with in Monzo.