Are we demonstrably getting better open source software? By what metric? (Not sure myself, asking in good faith.)

I sure don't see it. When I look at how fast FOSS advanced between the late 90s and the mid 00s, and compare to after the '08 meltdown, it doesn't look the same at all.

When you have small domain, any progress can be seen as big. Now when you have many millions of lines in linux kernel, half a million new lines is "oh, just another module". The same with whole foss domain. It's so big now that progress is seen as slow. Comparing Blender now to Blender in 2014 will show you how big progress we still make, just as an example.

Another module in linux isn’t half a million of lines. A whole new class of hardware with all the drivers may be.

Arguably by the metric of my own annecdotal evidence.

"Are we demonstrably getting better open source software?"

Yes.

The friction to create something good and useful is as low now as every. Anyone can create a github account, share source and overnight millions of people can use that thing. It happens here on HN frequently.