For something to get traction, it has to have excellent UI and UX. So far open source hasn't been very successful at providing that since there is no economic model to finance the immense amount of work required for good UX.
Regardless,
* Blender
* Godot
* Krita
* GitLab
* Olive
* Firefox
* Thunderbird
* NextCloud
* Amarok
* Vital
* and more
The problem isn't an "economic model". The problem is that most open-source projects simply don't attract UI designers, UX researchers, and users because contribution flows are more technical or geared toward developers.
UX research isn't "expensive" when referring to open-source projects because it predominantly requires people: designers, researchers, and users. The whole foundation of open-source is volunteerism.
Most developers I've worked with don't exactly love going back and forth with designers on UI stuff.
And volunteerism was much more popular in the 1990s when it was easy to write software by hard to sell it. App stores suffocated open source by offering a path to revenue.
It got beaten by FB Marketplace because Facebook has almost three billion captive users, and a marketplace bundled in the service they use many times a month takes less effort to visit than a completely separate site.
From this, and many other cases - like e.g. everyone hating the IM/videoconference system they use and yet using them anyway, I conclude two rules of thumb:
- UI that's good for users has very little to do with what UI/UX specialists peddle today;
- Network effects trump UIs - a sticky service with bad UI will beat a competitor that offers much better UI, but doesn't hold the user captive.
Had they not fiercely destroyed all the various "Search all Craiglist" mechanisms, they might have had a chance at fighting FB Marketplace.
Just the other day I had someone, an old person, complain about how a "revamp" of an internet banking site. They rewrote their functional, working UI, with a site that was designed by "UX designers", and you know how he phrased his opinion of the new site? "They made the site for stupid people".