Just use android if you care about things being open. Apple products are good because of the walled garden. Most people just want things to work out of the box, not to tinker with options.
Essentially, Apple gave up the right to assert this argument when they decided to abuse their control with iMessage as an explicit way of locking users into their system (which is just one example).
If I got sick of Windows, I could easily get rid of all of my Windows installations. But some of my games, even ones I bought and paid for, are not available on Linux. That is part and parcel a consideration I have to make regarding my choices. There are positives and negatives in every choice, and sometimes the negatives outweigh the positives. It's frustrating, and sometimes to make a choice you have to give up things you like, but that's life. Not getting to use FaceTime is not a significant cost, it's an emergent property of "I choose not to use iOS devices".
I do concede however that the "default messaging app" be just another "slot" in the OS that I can configure any way I want, no dark patterns a la Edge on Windows 10/11. But then again, I don't think iMessage has a URL handler anyways, so really it'd just be a few additions to the contacts card so I can share some link/picture/whatever to a contact's WhatsApp handle instead of their iMessage handle. Thankfully, the law in question from the OP does seem to shift us towards that future. Apple's walled-garden is fine, but if you play by the rules in that walled-garden, there shouldn't be favoritism for the gardener's services, and the gardener shouldn't be able to abuse their position to push their services either.
I'd personally prefer to just break up the gardener's company so their TV and Music streaming arm is a separate company.
https://arstechnica.com/gadgets/2019/01/facebook-and-google-...
Also Google themselves currently don't actually use RCS either, but rather a proprietary fork of RCS which prevents interop, and already refuses access to third parties who ask to interoperate. And app-store warfare does not address this at all.
https://arstechnica.com/gadgets/2022/08/new-google-site-begs...
https://ianbetteridge.com/2022/08/19/please-wont-someone-sto...
Trading proprietary iMessage for proprietary Google messaging is a net loss.
https://www.aljazeera.com/features/2023/5/9/facebook-has-3-b...
There is probably a substantial number of users who are content to use Facebook occasionally on desktop, and not even need it as a mobile app. The era of check-ins is far gone.
I would concede that Instagram is a different story, but even that platform isn't growing as rapidly as it used to.