Not if you actually fully or almost fully use the bundle. If you fully use your bundle every month you're saving money. You'd save more money than if you go unlocked phone + SIM only subscription.

But that is the problem partly: if you have a bundle, you want to use it, because you're paying for it every month. You're also bound to the contract (with its vague FUP), and the phone (for which you pay every month).

SIM only is only a subscription (often for at least one year) but in essence a subsidized phone is a loan. You bought something on credit. You pay for it every month for one or two years. I don't want that inflexibility!