It's not just pushing for it -- Java on Power Macs has already had at least one exploit, and PowerPC Flash already isn't supported by many sites (including its own security concerns). There is no comparable open source substitute for either. Plugins, like all proprietary binary blobs, need to die.
My favourite games on my G5 are Tron 2.0, Unreal Tournament (the original, using Westlake's OS X native beta), MAME, a recompiled version of Rise of the Triad and various ports of PrBoom. I'm trying to get Return to Castle Wolfenstein working correctly, but my build from source code chokes on the cutscenes.