Attack surface reduction is the important part. I’m not in disagreement with what you said, but if you took a modern iPhone and removed all capabilities other than sending and receiving phone calls, it would be much more secure than one which supports mms, email, browsing, etc.
Are you sure having a phone in your phone is a good idea? Phone calls are a significant source of attacks now, even if none of those attacks exploit a vulnerability in the phone software. As far as I'm concerned, the only point in having a dial-able phone number is to ensure I'm still eligible for car warranty scams and 2FA code harvesting attacks.
