Firstly, the discussion at the front about the music industry is quite insightful, but I'd go further - they didn't want to sell convenience at first. Used to the physical embodiment of recordings rather than the de-materialized reality of digital music, they spent years working on mandated inconvenience through DRM and other legal actions. Always remember that if the music industry had won unopposed you'd have to pay a license fee to set a MP3 as your ringtone, as well as having to license every copy on every device separately.
Secondly, about mongodb: I'm reminded of https://www.gwern.net/Complement "Commoditize Your Complement", but in reverse.
Open source software is by definition commoditized. Everyone can have a copy at no cost. So what is an "open source VC funded" company doing? Creating complements and selling those. It's just that there's no intrinsic reason that the same company that gives away labour as OSS should also make the best complements to that OSS. It's not a law of nature that e.g. Redhat should be the best people to provide consultancy on RHEL.
The music industry did not expect their product to become free to distribute at the margin, but every company operating as an "OSS startup" has to know from the start that their software can't be charged for directly.