We don’t possess Medium, true. One piece of it we could own would be free implementations of its editing interface.
I’d draw some more distinctions here. Medium is not just a writing tool. It’s also a publishing tool, a forum, and an archive. There are decent free versions of those. It’s also a commercial publishing company, and a ‘community’. The challenge is in bringing all of that together.
Medium works (to the extent that it does) because people want a system that integrates this stuff in a well-designed way. This integration work is difficult, so it takes full-time professionals.
An author can use any pen with any sort of paper. But even a professional programmer will probably have a challenge combining WordPress with some alternative editing interface.
We have many decent individual programs that deal with writing, editing, formatting, publishing, drawing, etc. I think the big deficiency shows up when trying to wrangle these into a coherent workflow. Something is wrong with operating systems. Ordinary users should be able to combine together simple, useful tools. So tool-developers shouldn’t be inclined to build up feature bloat.
(I know this isn’t my original new idea. The Unix philosophy, as an idea, wasn’t enough to secure that goal, so… I’m looking at other worlds. Most recently, Smalltalk systems.)