I would have paid a licensing fee to continue using it, but we've taken the time to switch over to Postgres now.
EDIT: Also, re: metrics of success, the examples listed as wrong there are _exactly_ why I enjoyed the product and was willing to invest time learning and implementing in a relatively new technology.
They didn't have a poor market; they didn't have any market at all. If I'm the CTO of XYZ, explain to me what I'm buying. I hate to keep coming back to it, but that's what Cygnus did so brilliantly: "You need an embedded development system. The existing solutions do nearly what you want. Pay us slightly less, and we will make GCC do exactly what you want." I know what I'm paying for.
"Build it and they will come" does not work in the open source space -- because if you have already built it, they don't need to pay you. And if you haven't, then they don't want it.
Now, if they had built a DB similar to Mongo and then sold a service that migrated people off of failing Mongo installations and onto RethinkDB, then they would be on to something. Tired of having your DB chew your data for breakfast? Want to have your reports finished the same day you ran them? For the price of a single developer, we will fix all your problems by migrating you to RethinkDB. That's something that will make money.
But who wants to charge for services? How will we get our multiples if we only charge once for our work? That's just folly!