The default messaging apps on iPhone is iMessage. It’s locked down and can not communicate with any other messaging app except via SMS. Therefore the other apps have to use it to communicate with iPhone users, who you will never, ever convince to download a third party messaging app
One other thing is that none of the third party messaging apps can even use SMS. iOS is designed so that only Apple can use SMS.
I suppose it’s possible to build a system that would let you specifically allow a VPN IP to be green-listed on your account, but you’d probably have to allow it by signing in from a known good IP first.
I think it seems like lot of work for something that isn’t really private and is still probably vulnerable to exploit.