> significant overhead cost for anyone who needs to handle images from unconstrained sources

Flickr is doing just that, and they’ve been using GPUs for more than 2 years already:

http://code.flickr.net/2015/06/25/real-time-resizing-of-flic...

> It'll happen but this kind of thing always takes longer than it seems like it should.

I think the main reason for that is lazy software developers reluctant to learn new stuff.