Its never one platform - thats a start. Most people use whatsapp for sure, but some prefer messenger, some instagram etc.
Whatsapp (or signal) is more advanced compared to imessage. Sure, some Apple's specific features are missing (like live stickers apple introduced at WWDC) but I'm sure those will come eventually too.
As for quality - well, usually its not an issue as its among friends/family BUT if you want high res whatsapp (for example) introduced hires version so nonissue. I think Signal might have that option too but haven't checked.
There is a reason why most of the europe is on whatsapp - also, in europe we have GPDR (or whatever the shortcut is) that protects us. You have no idea how much spam I've started receiving when I moved to North America. Its crazy how unprotected you are here as customers.
People bitch about EU but EU does a lot of things to protect customers. You can clearly see it with food when NA has chemicals allowed which are banned in EU.
In general, the food quality in EU is 100x better than here in NA. Really don't understand why NA allows so much chemicals to 'poison' its own food. Mindboggling
Hmm, honestly I don't know enough about WhatsApp and iMessage interoperability so maybe you have a point. One sticking point for me is picture/video quality, is this an issue WhatsApp to iPhone? And I mean directly sending through texts, not sending a link. I'd also be curious how group messages truly work as you note, are they truly as seamless as iMessage to iMessage, and do you get all the other stuff like tapbacks, typing indicators, etc.
Finally, it's not about installing an app, it's about coordinating all your contacts to use one service. I have hundreds of contacts, it would be an absolute nightmare to try to get them all on WhatsApp.