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.