If you fail to pay, the wallet'd simply downgrade to a free one, with all keys and signing abilities intact. There's just no advanced collaboration features enabled. You'll still be able to sign, export PSBTs, send them to other co-signers to collect signatures (using out-of-band channels such as Signal), broadcast transactions, etc.

And in the absolute worst case where Nunchuk completely disappears for some reason, you can always recover the wallet elsewhere, such as Sparrow: https://nunchuk.io/blog/wallet-recovery-sparrow

The whole thing was designed to never have any single points of failure.