I think folk are overthinking this. A pot has to belong to someone - if I set up a shared pot, then I assume that it’s part of my account, and that I’m giving others permission to pay in, view (and maybe withdraw) money - but legally it still sits on my balance sheet. If I die then it’s just treated like the rest of my assets.
Same as if I’m in the pub with my mates and we start a kitty. Whoever’s holding it could drop dead (or run off, whatever)…
I’ve no insider info, but anything else would seem (to me at least) to be significantly overcomplicating things. Maybe when shared pots were mooted as a solution to joint accounts, then there could have been a push to some form of shared liability, but I think the general outcry of “we want normal joint accounts” has closed that door (and possibly some more inventive solutions 
).
Anyway, just my views. Monzo haven’t officially announced any joint pots - or the problem that they would fix by having them yet. So this is just speculation - and others will likely have very different views. 