This guy offers some interesting points.
I am a (small) investor in Monzo and agree with him. Monzo seems to be drifting rather than developing a sustainable business model. Charging £5 per month for a shiny card and features you can get for nothing with high street banks (interest, open banking, fee-free withdrawals) isn't going to drive profitability. Nobody except a few hardcore fans is interested in that sort of deal.
They seem to have a strange block on acting like a "normal" bank and doing what normal banks do to turn a profit, i.e. lending out deposits. If they want to survive, ultimately they are going to have to offer credit cards and loans and mortgages like normal banks do. I don't understand what they have against it. It's almost as if they think lending money would make them bad people, rather than providers of a service that most of us need at some point in our lives.