Is it though? Upward mobility is a long-term trajectory. Sustaining a profitable career for 10, 20, 30+ years even as an independent software contractor is extremely difficult (especially as the market floods with competition). I think most devs who jump into the freelance world only see the short term hourly wage and miss the risks/opportunity costs involved with running a business. Those risks and opportunity costs will manifest themselves in the future.

Stagnation on a higher level is much less of a problem than stagnation on a lower level. But the much bigger difference is that software contractors usually decide to contract, whereas warehouse contractors and the like do contracting work because they would not be hired as permanents, the decision happens mostly on the hiring side.

I was about to make this exact argument, but on inspection, the authors of the article are making a more subtle point. Their claim is that extensive use of even "white collar" contractors may cause long term harm to organisations, projects and staff.