You're always in someone's walled garden. Didn't someone get their domain seized by Cloudflare the other day?
There is no technology in the world that isn't vulnerable to this sort of thing. The only way out is legislating against companies cutting you off without reason or recourse.
Indeed! One has control on their DNS records only as long as they are allowed to have control on their DNS records. That control can be taken away as it once happened in my case when a domain name I was using was sinkholed due to a false positive. Full story here: https://susam.net/blog/sinkholed.html
Until the sinkhole incident happened, I was a firm believer of running my own email server with a domain name I have registered, so that it would not be possible for a large corporation to accidentally lock me out of my email. But now that I have seen how one's control over a domain name can be taken away suddenly and without warning, I am not so sure!
My XMPP address will keep working as long as I have control over my DNS records.