In the case of GDPR it is not just for simplicity. It's because companies that operate in the EU need to provide those protections to all EU citizens, even those across the pond. You cannot check if someone is an EU citizen so if you operate in the EU you effectively need to treat everyone like an EU citizen.
Not really as a design change as drastic as user exchangeable batteries means phone companies would probably rather adopt a unified design (removable batteries) than a region based design
2027 is actually pretty early for such a dramatic change, and somewhere I heard that it's all phones sold, if that's the case (i.e. you can't sell old models if they don't have easily replaceable batteries) than that is a really early date for such a law.