Earlier quoted context omitted.
If you have your own server (of any kind, even a Raspberry Pi), there's no excuse for not running your own CardDAV, at least (you can also run CalDAV), and it works beautifully: https://www.stavros.io/posts/private-contacts-and-calendars-... Radicale even commits every change to a git repo, if you want, so you can go back to your contacts history an arbitrary amount of time just by using `git log`!
I'm interested in keeping my data private, and willing to pay for it, I'm just not interested in maintaining a server, and dealing with 1) keeping it up, and 2) keeping it secure. I'd just rather pay someone a small monthly/annual fee to do it for me, along with keeping my data private.
Edit: Not saying it's modern or user friendly. It has worked for me for 10 years without touching it, though.