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Because the most useful communication apps are the ones that you can reach people on. XMPP’s lack of user friendly UX or long term support and commitment make it DOA for most normal people, which in turn makes it DOA for everyone who might want to talk to one of those normal people who are turned off by it.

I think OP is overblowing things, and is especially misguided in recommending gmail, but at the same time, they do have a valid point and I think you’re somewhat misrepresenting what they said.

For one, they specifically said that the proton domain email addresses are problematic (protonmail.com, pm.me), and weren’t talking about custom domains that sit in front of Proton mail.

For two, their point is valid. Auto-forwarding being paid, does create vendor lock-in and make it hard to switch away from Protonmail if you use the OOTB addresses. It’s something worth considering.

As you said, the recommendation should be to use a custom domain that sits in front of Protonmail rather than switching to Gmail, but paid auto-forwarding is a valid criticism.