And I'll be the contra to your take: the iMessage ecosystem is so closed that everyone without iphones can barely even interact via sms with iphone users. This is overall such a huge problem that it makes the closed ecosystem security solution not a practical solution
Nope, SMS works. Just fine. Apple rightly warns you that you are in a lower security environment by adding a bit of visual friction. This is true EVEN IF you are using RCS, because European laws require termination and inspection when RCS messages “interoperate” and are sent to other providers.