> In contrast, I still have to pay a fee to send SMS messages to my European relatives, so there's a financial incentive to overcome that initial friction to switch messaging provider.

In addition to the financial disincentive for SMS in the EU, communications iMessage to iMessage are E2EE whereas SMS messages are not, even in the context of iMessage to SMS and vice versa.

Also also, Whatsapp’s E2EE is not as reassuring given Meta/Facebook owns Whatsapp.