You’re not getting my point, probably because I’m finding it hard to explain.
If you think about a legacy bank, systems are wrote and process is put in place which takes a monumental effort to change/fix.
Often they can’t do something simple because some code on a mainframe written 30 years ago that’s now tangled into a complex web of crap can’t be changed without huge risk.
Not being able to close a Monzo personal account when I only use a joint account feels very much like that scenario in the sense they are coupled together.
Like you say in your original post… It’s just a byproduct of the way joint accounts were set up and Monzo probably didn’t set out to do it this way as they weren’t aware of the joint account demand. What feels un-Monzo like, is they haven’t thought and catered for the need to have only a joint account when offering joint accounts.
I don’t really care to debate why its turned out like it has. This is a suggestion for improvement.