in 2016, 24.1 million people in England and Wales who were married in 2016; this was 50.9% of the population aged 16 and over.
& since Google says there’s 65.64 million people living in the UK & 18.8 million are under 16 (ONS), roughly half (Monzo’s only available to over 18 year olds) of the population aren’t married.
That’s your subset of Monzo users who presumably don’t need the legal benefits of joint accounts.
We’ve seen that Monzo tends to release features cover the most common use cases, before iterating on the design later. That’s meant that users who don’t get paid on the 1st of the month or have the Android app, haven’t been able to use / had access to, all of the latest functionality straight away.
Since shared pots cover bill splitting for flatmates, users sharing expenses on holidays, parents managing their children’s money, couples who aren’t married etc. my guess is that this is the core functionality that will be released first before the legal status is added later, assuming it will be.
Obviously the legal status is a common use case & Monzo are aware of it so maybe it will be available in the first version of the feature. But you can imagine that it could add complexity..
In the meantime, users would still be able to also use their legacy bank’s joint account - as some Monzo users presumably do at the moment.