While that was the case ten years ago, honestly the macOS app ecosystem is mostly running on fumes at this point. There's few people coming into AppKit development, and iOS developers seem to have an irrational fear of putting any effort into learning the desktop paradigm even though the API is largely the same.
Apple is making half-hearted efforts to fix this problem by introducing two new GUI APIs on the Mac. "Catalyst" is a porting layer that lets you put iPad apps on the Mac desktop. The look'n'feel of these apps is pretty much as clunky as you'd expect. Then there's "SwiftUI" which is a new React-like runtime that spans all Apple platforms. SwiftUI is in its early stages and will take years to catch up with AppKit's functionality.
At this point Mac desktop development is effectively in a limbo: no one wants to start new AppKit projects because Apple is strongly implying that it's deprecated (although they don't seem to know exactly what it's being replaced with). So it's pretty much Electron or Qt on the Mac now, unfortunately. As an AppKit developer since 2002, it breaks my heart a bit.
I don't have data, but there's no way that the Mac developer ecosystem is worse off than it was in the 00s. There's significantly more Mac users now, and there's orders of magnitude more developers with experience developing for Apple platforms.
Maybe most devs won't venture outside of iOS to try Mac development, but 10 years ago the few long-time Mac devs were (by necessity) putting their Mac projects on hold to work on iPhone apps.
> because Apple is strongly implying that it's deprecated
Lots of iOS developers are writing blog posts claiming things like this, but every single Apple app on the Mac is written using AppKit. You can't deprecate the technology behind your entire platform.
Maybe 15 years from now SwiftUI will have replaced AppKit as the dominant way to write UI code on the Mac, but generations of apps will be born and die between now and then.
I don't have hard data either. But ask any old Mac developer on Twitter whether they're doing better now than ten years ago, and I bet a majority would disagree.
For one thing, the rise of mobile app stores has destroyed the perceived value of software. A $50 app now seems very expensive to most, whereas it was mid-priced back in 2008. Yet the Mac App Store has failed to bring in the mass audience that would compensate for the lower unit prices.