(is fully closed source software development even still a thing? is there any popular propriatary programming language / editor / runtime / ecosystem?)
However Visual Studio and Xcode are closed source and still popular in some circles.
Most people will still be relying on open source libraries while using those platforms though.
Copyleft free software licenses such as the GPL explicitly grant anyone the right to use the software for any purpose, as long as they also extend this right to their own software. The intent of this license was to infect any novel software built upon GPL-licensed software, forcing it to become free as well; in practice any organization who wants to build a proprietary app or service simply avoids GPL dependencies (or blatantly violates the license terms). Empirically, software companies care more about avoiding being forced to release the source code of their own proprietary software more than they care about using the exclusively-GPL'd software commons as a dependency, and this isn't a problem the license itself can solve.
I'd suggest using AGPL instead of just GPL.
Conclusion is that gpl but only for non-commercial does not work. They need to use a different license to get something self-consistent.
I don't think it's that bad. Most commercial enterprises don't want their commercial products to be under GPL terms, so they'll pay for the license regardless. You should really fix the SaaS loophole by using AGPL instead though.