Proposal:
Remove "Religious differences strain your relationship." and "They are spreading their own religion, but you converted some of their cities to your religion." diplomatic penalties if that founder no longer has any city with the founder's majority religion and does not have any great prophet/inquisitor/missionary of the founder religion.
Rationale:
It's a sunk cost fallacy for the AI to still hold a grudge if you have no way of regaining your religious benefits and in fact you should welcome being converted than more so just like a non-founder. You might get world religion benefits for instance or at least any benefits.
The very niche ways of still being able to conjure back your religion however briefly like getting a great prophet through leaning tower of Pisa or recapturing a stolen missionary are not worth the likely inevitability of actually losing the religious race in the long run.
Remove "Religious differences strain your relationship." and "They are spreading their own religion, but you converted some of their cities to your religion." diplomatic penalties if that founder no longer has any city with the founder's majority religion and does not have any great prophet/inquisitor/missionary of the founder religion.
Rationale:
It's a sunk cost fallacy for the AI to still hold a grudge if you have no way of regaining your religious benefits and in fact you should welcome being converted than more so just like a non-founder. You might get world religion benefits for instance or at least any benefits.
The very niche ways of still being able to conjure back your religion however briefly like getting a great prophet through leaning tower of Pisa or recapturing a stolen missionary are not worth the likely inevitability of actually losing the religious race in the long run.