Maybe more than once, to make it harder for scammers to dismiss notifications and pretend they've installed a genuine battery. But yeah, a finite number of warnings, and don't brick the thing.
If delays between warnings are long, the scammer will wait until one is shown and then sell the fake, and the buyer will only be warned when it’s too late.
If it’s high, the difference with “forever” is small, and you’ll still be annoyed.
Having a separate screen that shows the warning on demand or showing the warning only after a new sim is inserted might be the best option, but I fear too few customers would know about it.
People learnt to check iCloud lock status before buying an used/stolen iPhone.
Grandma don't know how to access diagnostics? She can and should ask her techie grandson for help when buying an used phone.
There is absolutely no excuse for this purely anti-competitive behavior.
FWIW I'm not an Apple hater, I think their laptops and phones are the best out there.
But I have an android instead of an iPhone because I can't stand not being able to install what I want in iOS, that's a deal breaker for me.
And if they go the same route with MacOS I'll switch to Dell+Ubuntu.