As someone using RethinkDB in production for the last 6 years or so: I am really disappointed by how software development world is dominated by fashions and fads.
RethinkDB was done really, really well. It is one of the very few distributed databases that went through Jepsen relatively unscathed and delivered on promises made. Development was done in the public, questions were asked through StackOverflow. You interacted with competent, skillful and experienced developers.
And yet MongoDB was the latest fashion fad, end even though it did NOT deliver on the promises, it was the hot-database-du-jour that kids used.
The article is mostly about the business model, and I also always thought they would have a hard time making money on the database. But I think they would have had a much better shot if it became more popular. It didn't, which accelerated the company's demise.
EDIT: I just realized that I wrote about RethinkDB in the past tense, even though it very much exists as I write it. In fact I run my business on it. But because it fell out of favor, when it was open-sourced, it failed to pick up momentum, and it now seems unlikely that it will.
Which is why I'm working on switching to FoundationDB, and I'm slightly worried that it will suffer the same fate: it is excellent technically (the best transactional guarantees in a distributed database you can get), but difficult to understand and not very user-friendly. It's not the "node.js database for everyone". The only reason I'm considering it is because Apple uses and develops it, which gives me hope for longer-term maintenance.
Going back to fashions — you can have a product which excels technically, but if it's out of fashion, it might as well not exist.
I think we would all be better off if we stopped trying to always pick The One True Database, The One True Programming Language, etc — and instead accepted that there might be multiple tools, each specialized for certain kinds of tasks.