Out of these millions of web developers, regardless of which stack they use Javascript is the one thing they have in common whether they love or hate it.
The lines between web and desktop are blurred to a point anyway, and there are plenty of people out there with experience developing rich applications in JS.
I'm guessing the play here isn't so much to get people to write gnome3 apps from the ground up but to focus on making it possible to give rich web apps first class gnome3 support with the minimum of extra code.
Of course other bindings will continue to exist for those who prefer to use them.
This move is bad precisely since a lot of web and native developers alike realize that Javascript is a terrible language, and rightly hate writing it. This move serves to alienate all those devs who would rather write in a sane language since desktop development gives them that choice, but instead see GNOME embracing the biggest wart of web development to date.
The same arguments could be made to PHP or Visual Basic and I remember people saying nasty things about the C++ WinAPI back in the day.
Unless you have the luxury of being Apple you don't really get to say "here's our blessed language , you will learn it" if you want to appeal to the masses.
Granted, hence why the GNOME devs didn't choose Vala. That doesn't explain why they didn't choose a cleaner, easier language, and it certainly doesn't explain why they chose the ass-end of web development.
> already know
The developers that Already Know only Javascript are not, in my opinion, the ones that GNOME will garner the most benefit in attracting.
From a business point of view you are likely looking at a considerable time sink on an existing developer, or possibly hiring an extra one.
This to develop for a platform with ~1% market share does not necessarily look like a great investment.
Javascript on the desktop allows you to take a bunch of the code that you may have already written for the web or for Metro or for phonegap , add GTK stuff for desktop integration and call it good.