>I believe that the availability of the software's source code to its users is unrelated to the fact of whether the actual development of such software is paid or not.

This is true, and I've seen that happen too, one of the companies I worked with would sell source licenses to customers, and as I understand it this was very common in the mainframe business going way back. The software vendor retained rights to sell and distribute the software though, that's what I mean by proprietary.

I've seen these arguments for open source and libre software before many times, but there's a huge discontinuity between the theory and what actually happens in reality. In the real world there are tens of thousands of small and large software houses producing niche software for diverse use cases for businesses all over the world. Hundreds of niche engineering design, test and optimisation applications, B2B services, audio and video tools, chemical engineering tools, automation and industrial control systems, booking and billing systems, here are an almost infinite variety. Most of them are only known to people actually in these niche specialisms. In that world customers paying for customisations is stock in trade, it's entirely normal. In fact the company I'm at right now is paying the vendor for customisations to our incident and change management ticketing system.

In comparison open source, outside nerdy IT oriented tech projects, might as well not exist. It's minuscule. Barely even a footnote.