I'm not sure some of the users actually knows the state of the FOSS project. I would bet there are more than a few businesses that have software they paid for that has some code from these projects.

If it works well enough that nobody has noticed support has stopped... Maybe the software is done?

In general, I would agree, but you are one OS upgrade from failure (see Mac OS Catalina). I would imagine some testing will surface that the company needs to act.

I find this is quite difficult to do in practice.