That sounds like the 90s argument against FLOSS: without the incentive for people to sell software, nobody would write it.

Software wasn't always covered by copyright, and people wrote it all the same. In fact they even sold it, just built-to-order as opposed to any kind of retail mass market. (Technically, there was no mass market for computers back then so that goes without saying.)

That argument seems to have been proven basically correct, given that a ton of open source development happens only because companies with deep pockets pay for the developers' time. Which makes perfect sense - no matter how altruistic a person is, they have to pay rent and buy food just like everyone else, and a lot of people aren't going to have time/energy to develop software for free after they get home from their 9-5.

Without IP protections that allow copyleft to exist arguably there would be no FOSS. When anything you publish can be leveraged and expropriated by Microsoft et al. without them being obligated to contribute back or even credit you, you are just an unpaid ghost engineer for big tech.

This is still the argument for software copyright. And I think it's still a pretty persuasive argument, despite the success of FLOSS. To this day, there is very little successful consumer software. Outside of browsers, Ubuntu, Libre Office, and GIMP are more or less it, at least outside certain niches. And even they are a pretty tiny compared to Windows/MacOS/iOS/Android, Office/Google Docs, or Photoshop.

The browsers are an interesting case. Neither Chrome nor Edge are really open source, despite Chromium being so, and they are both funded by advertising and marketing money from huge corporations. Safari is of course closed source. And Firefox is an increasingly tiny runner-up. So I don't know if I'd really count Chromium as a FLOSS success story.

Overall, I don't think FLOSS has had the kind of effect that many activists were going for. What has generally happened is that companies building software have realized that there is a lot of value to be found in treating FLOSS software as a kind of barter agreement between companies, where maybe Microsoft helps improve Linux for the benefit of all, but in turn it gets to use, say, Google's efforts on Chromium, and so on. The fact that other companies then get to mooch off of these big collaborations doesn't really matter compared to getting rid of the hassle of actually setting up explicit agreements with so many others.