As many people have already written, it's not the drive to tablet and phone that is reducing the user community, it's the fact that Gnome has become so bad compared to other DEs that people moved away. The main question is "why has Gnome become so bad?". I'd say it's mostly due to not listening enough to user feedback and lack of good judgment on what is good for the users.
Don't get me wrong. I loved Gnome, used it all the time, even used to send patches for the bugs that were annoying me (actually, I even had SVN commit rights at some points). But I stopped because Gnome 3 was worse than Unity _and_ LXDE, and because developers started to close all my bug reports as WONTFIX or, worse, because the patch would not apply anymore... after 2 years of being ignored.
I'd suggest these changes to all the core Gnome developers:
* first fix bugs before adding a new feature (or a new app)
* review and merge as many patches as you get from outside people, as soon as possible (that's how you build a developer community)
* review the entire interface and especially the fixed/default values so that Gnome is _super_ comfortable to use right out of the box
* do not ever remove features, and never accept regressions
* make sure your interface can be used by power users too (yes, that means putting back _some_ configuration options), they are the (future) developers
* listen a bit to user feedback (that one is difficult because it's typically a very noisy channel, but it's necessary)
* pick a few known and powerful programming languages, and stick to them for all the core applications. Honestly, just drop Vala: as great as it could be, it's not up to a DE project to develop a new programming language, and almost no one outside of the community knows it. If it was up to me, I'd say, just pick C, C++ and Python.
Keep like this for 3 years, and Gnome will be relevant again.
I'd also suggest to pick 2 or 3 apps and focus on them so much that they are the best for the task among any other competitor. This way, people will have incentive to use Gnome, and all the distributions will make sure these apps and all the dependencies are installed by default and working well. For instance, I'd pick: Evince, Rhythmbox, and Aisleriot.