#FullMonzo does not mean not having a legacy bank account.
This is a huge bugbear of mine. It means using Monzo as your main account and considering it as your main account, i.e. getting your salary paid in and your bills paid out.
The full monzo badge requires a CASS switch, but even if you switch manually and keep your legacy bank account open, you are still full monzo, just without the badge.
No-one at Monzo has ever recommended only having one bank account. Always a good idea to have a backup. (Ideally one with Visa, one with MasterCard).