On
http://webglstats.com/ with about 7-8 million visits across its contributing sites, I measure 90% desktops and 10% mobiles. For much of 2012 and 2013 the trend for mobiles has been rising, but in 2014 it's flagging somewhat.
http://gs.statcounter.com/ has Desktops at 62% and mobiles at 30% (the rest being uncategorizable other things). The change from last year is desktops -12% and mobiles +14%. Even assuming the trend'd continue linearly, you'd look at years before you could "lay desktops to rest" as an audience.
However I'd also not be happy with a situation where a machine that I can stick in a GTX-980 that'll get about 1000x as much performance as any of the latest smarphones, is something that can't run WebGL well. I think that's counterproductive to WebGL.