Yes, it is not popular, for the reasons I already mentioned.

What puzzles me is why so many HN comments, including yours, frame this purely in consumer terms: "If this open source tool doesn't meet my needs, I'll switch to a proprietary one."

And that is perfectly fine. Use whatever works for you. No issue there.

What seems misplaced is the expectation that Matrix must be popular. Why should it be? It is not your project, and you are not contributing to it. Where does this expectation of its popularity come from?

Matrix already serves its developers and contributors. If it does not serve you, you can either help improve it or choose a proprietary alternative. Both are reasonable paths.

What feels off is the dismissive tone suggesting that if Matrix is not widely adopted, something must be wrong and proprietary options are therefore superior. In reality, this is just how open source works: projects exist to serve those who build and support them, not necessarily the mass market.

There is nothing wrong with an open source project not meeting everyone's needs, leading some people to choose proprietary alternatives. Remarks like "This is the fastest way to get people to say: I hate proprietary solutions but at least they work" or "OK great. I guess you answered why Matrix is not more popular" are not really the decisive critique you think they are.

Open source and proprietary software each have legitimate roles. For some use cases and users, open source tools are a better fit. For others, proprietary solutions make more sense. Popularity alone is not a meaningful measure of value and choosing what works best for you is entirely reasonable either way.