It’s not like even the esim happy us carriers won’t provision a phone using a sim. There’s no reason you can’t use an older device with a sim as a backup for a newer device with an esim.

I think (but I need to confirm) that you can have an esim and matching sim at the same time. That was the process for going from sim to esim, they’d make an esim with the same ids as your sim then you make sure the esim is working and chuck the old card. Gotta make sure it can go in reverse though.