It's kind of insane to run a business on free software. If your CalDAV server suddenly has a bug with a partner who uses Apple/Windows/etc calendar, what are you gonna do? Drop everything else to start debugging? Call and tell them to switch calendars for you? Abandon them?
Card carrying FSF member here, but business is business.
If you were a small lawn mowing company and happened to receive a free mower for your operations, you wouldn't just run it into the ground without regularly paying someone to keep it in good repair (though this analogy does raise some interesting thoughts about companies in general and their maintenance policies).
In exactly the same way, you shouldn't be using software of any kind, paid or not, without having some plan for maintenance and repair.
If free software can cover the same business needs as paid software, there's no reason to not use it. But you absolutely cannot sacrifice maintenance just because you did or didn't pay money for the software itself.