The GPL 3 is a pretty benign improvement on the GPL 2 (though I know Linus objects to the Tivo clause). I don't think anyone who was happy with their software being used under the terms of GPL 2 would be unhappy with it being used under the terms of the GPL 3 additionally, rather than exclusively.
But let's imagine the next version of GPL 4 was "Additionally the authors may at their discretion use it under the terms of the CC-BY-SA license". Nothing wrong with it as a license, but that's a bigger leap in terms of license changes.
It's not hypothetical that they could exercise that kind of power - they _did_ with the GFDL with the "wikipedia can relicense as CC-BY-SA" clause in GFDL 1.3.
I think most people would agree that that was a responsible use of that power. but I can understand those that don't want to extend the trust that all future uses will be responsible.