Yea, in my opinion thats also the actual problem here. World leaders using run of the mill iphones poses a real security risk. They should be using some hardened ultra secure device or other communication methods.
What we have now for work devices with manageable profiles seems to me to be a good balance that should possible to harden enough for extreme cases as well.
On the other hand "run of the mill" phones that you can hand to a teenager (or use as a backup phone) don't need ultra-security and the cost that comes with it.
Really low end devices would probably be secure by simple virtue of not having enough features to enable hacking.
Ideally, you'd want to build your own phone where you control the supply chain form hardware to software running on it. I do believe the US president has a custom NSA hardened phone.
Our phones are still pre-seatbelts, pre-crush zones, pre-lead-free gas, pre-standardized controls and signal lights, etc.
Our phones only meet electrical and rf safety standards. When one phone model one time had a bad battery, it was a big deal. But if someone steals your retirement, oh well.
Nearly all VIP's insist on at least have their private phone as well, if not simply refusing to carry / use the issued devices.
Maybe this is someone a large country could develop internally themselves? Create their own fork of Android, private app store, etc etc?