I rather suspect that the reason Bundles work while Customised Bolt-Ons don’t is because while Monzo may be able to arrange for the combination of A, B, C to be cheap, that arrangement doesn’t stand up when it’s A, B, X; A, Y, C; Z, B, C - or any other combination thereof.
I’m also reminded of the once hugely popular Humble Bundle software packs, where the majority of developers providing software were probably counting on people: (a) paying the default amount, (b) leaving the default split; and therefore probably hated people like me who would assess the bundle, figure out how much I was comfortable paying, and then carefully moving all the sliders to make sure the money went to where I wanted it to go - which also meant moving plenty of sliders to zero for the items I either couldn’t use (no OS version), didn’t want (am old enough to know what game genres I never want to play), or already own (I’ve already given them money, but they can’t see that).
Replace ‘Humble Bundle’ with ‘Monzo’ and the games with the bolt-on features, and it’s easier to see why - from a commercial perspective, it may not necessarily stack up. And also considering why a company would rather be included in a mandatory bundle (where they have a defined and regular income) than a fully bolt-on solution (where they may never see an appropriate return on any investment they’ve made if not enough people pick them to bolt on).
It’s a complicated subject, I think I may be drifting slightly now and losing track of the general point I’m trying to make among all the detail.
tl;dr, it’s very easy to understand that commercial considerations favour one approach over the other. It’s difficult, though, to stomach being told that decisions made for commercial reasons are, allegedly, customer-driven decisions. We’re all grown-ups here, aren’t we? I’d be more OK with Monzo saying “We tried to disrupt the sector but it didn’t add up, so we’re aligning closer to the sector now (as the alternative is to dump it entirely)” than I am with “We believe this is best for the customer (really) and this is the only reason we’re changing it (honestly).”