Is it, though? I really don't think it has. There's still plenty of GPL-licensed software out there; most of the software I run on my laptop is still GPL, for example (the main notable exception being Firefox).
I think perhaps there's just a lot more open source out there these days than there was 10 years ago. A good amount of it was started by or sponsored by corporations, and sadly those types of entities are less likely to use the GPL.
Ok, I also have no hard data at hand, but in my impression the vast majority of projects even started by individuals today do not use the GPL, but rather MIT or alike. Me too, because I do not like the concept of copyleft. I like the intention behind it, a world with open source as standard - but I just think the way towards this, is by convincing people step by step and proofing that they can also make money with open source and not trying to force them to open source.
I think perhaps there's just a lot more open source out there these days than there was 10 years ago. A good amount of it was started by or sponsored by corporations, and sadly those types of entities are less likely to use the GPL.
But that's fine, no skin off anyone else's back.