Easy solution: comply. Let this ONE country be a very public guinea pig. Either:
- Nearly all iPhones in this country will be destroyed by everyone taking advantage of the new option OR
- Some iPhones in this country will be destroyed by some taking advantage of the new option OR
- Nearly nothing will happen and all of the spin about the certain cataclysm that will follow such an option will be seen for what it likely is: far, far overblown.
We ALL have Macs running almost the same OS. Those Macs can buy/install Apps from an iOS-like store completely controlled by Apple or Apps direct from third parties. Are all Macs completely destroyed by us all having that kind of store flexibility already?
How does Apple address this situation on Macs? They pop up a warning about installing apps from unknown sources. Then it's on the Mac owner to proceed or not proceed. If they proceed and the owner is actually installing every computer virus ever created, that's clearly on the owner. Apple could send a "we warned them" notice to itself so that when this person is calling Apple for help, Apple customer service will know that software was installed from questionable sources.
Has the flexibility to purchase Mac Apps from wherever brought all Macs to their knees? Not at all. Have some Macs been compromised by that flexibility? Yes. iOS devices will likely be the same. Most will probably continue to get apps as they do now. Those concerned about safety will continue to get their apps from what they believe is the safest source. Some of those less concerned or too dumb or naive may- in fact- compromise their iDevices.
Here's a chance to show the world how "terrible" it would actually be for a finite group of people to facilitate competition... just like all of us already have with our Macs. If it actually is a disaster, a slice of one relatively small group of people will suffer the consequences, learn from their mistake and not make it again. On the other hand, if iOS goes as macOS is already, the flexibility to purchase from more than one source will likely deliver better prices and more money actually reaching the developers instead of a company already richer than any other.
Before it's forced on Apple by many countries, here's an easy opportunity to prove the disaster... or reveal the hype is false. If disaster, other countries wanting the same may pull back and preserve the "as is." A very simple test will clearly prove it if it will be as spun.