We use amp-img instead of the img tag to be able to control when resources load (read not load them unless actually needed). Doing this with a different hack like <img data-src="foo"> is possible but a less explicit hack.
To your first point: It might have been just the right point in time, but maybe it isn't. I argue that if adoption is good users will win. It could totally fail, but then it was still worth trying.
In case you haven't tried, check out this video https://www.youtube.com/watch?v=i2_lAEzmOPo&feature=youtu.be I so much want my experience to be like that (or try g.co/ampdemo on your phone yourself).