You certainly made the case that the GPL was the only force, or at least ignored the contribution of alternative licenses.
I also wouldn't agree that proprietary software is in decline. There are niches where the OS, mobile apps, and games are almost entirely proprietary (and that is not changing any time soon). But the most damning problem is that all computer hardware now has multiple layers of subsystems with proprietary software components, even if the boot loader and beyond are ostensibly FOSS.
My take on the cause of proprietary software is "the bottom line". Companies want to sell products and they believe that it's easier to sell things that are not open source. Meanwhile, there are several counterexamples of commercial products that are also open source (not necessarily copyleft), including computer games. The cause of whatever decline you're seeing in proprietary software dominance is unlikely to be the GPL.
