Moving to WhatsApp takes out the Apple restriction, which is a step up as I can use WhatsApp on Windows and Linux too.
The drawback is the phone number requirement, but this is true of all the other chat apps. If there was one that was functionality equivalent but accounts didn't require a number then I would switch.
I can agree any Apple-specific lock-in is not good, however you're talking about the ability to chat on the platform and not going beyond just access to include whether we should be trusting our data to that platform—different questions. Both XMPP or Matrix have clients on all OSs, lack a phone number requirement, but can also both be self-hosted + decentralized, and with open specs and you can read the source on the encryption to validate the trustworthiness. These are the routes we should head and there's nothing stopping folks from using them today other than they don't have a for-profit entity to market them.
Obviously, this goes back to the root issue as there are philosophical reasons to straight prefer certain technologies—like the ability to side-load apps on your pocket computer—that you should try not to compromise on just because of peer pressure.
I agree, I dislike sharing any data with Facebook/WhatsApp but I shouldn't require children to spend $1000 to be allowed to communicate.
XMPP would be preferred, but isn't quite as simple as iMessage/WhatsApp at the moment, and Matrix isn't the same. I do see that some of the XMPP clients are getting pretty slick and I could probably move the family to something like that soon enough.
It's a bunch of XMPP software (Conversations, Siskin, Prosody), with minor patches, under a common branding (the Snikket parrot), plus a web portal.
It's aimed at the friends&family use case, with an easy invitation-based onboarding workflow. You can either self-host a Snikket Server (= Prosody + TURN + certificate automation) or sign up for the hosted beta (which is either bring-your-own-domain or a domain under snikket.chat).
Note that snikket.org is not, by itself, an XMPP service where you can just sign up. You either need to run your own instance, or you need to sign up for hosting.
I self-hosted ejabberd on an 2014 smartphone using postmarketOS last weekend. The defaults were almost all good enough and the only tricky part was setting up all the networking bits (getting my domain names hooked up to nameservers, dynamic DNS, Nginx proxy on the router to get ACME certs from the built-in ejabberd module) because these aren't thing I work with on the regular. Performance for users countable on one hand hasn't been an issue. Self-hosting Synapse for Matrix on an at-home device... good luck.
The drawback is the phone number requirement, but this is true of all the other chat apps. If there was one that was functionality equivalent but accounts didn't require a number then I would switch.