On one hand, you have a faster loading site/page which is good for users and helps with SEO ranking, but as a developer, you lose some flexibility in terms of specifying your layout for multiple cases.
I guess if the majority of your traffic is mobile-based in areas with typically slower internet connections, then it makes more sense to utilize something like AMP. But if that is not the case, it seems like you have to do twice the work for not much gain...
There is no reason why a normal website can't be as fast or faster than an AMP one, but AMP gives the framework to get organisations that otherwise wouldn't care enough to do so.
Besides, Google has announced that search rankings will be affected by speed, not by what library you use. So, if your site is already optimal (doesn't benefit from AMP) or you want to use some competing project that also does a great job, you won't be penalised.
I don't see the walled garden argument here.
I had jumped to some conclusions; if I am not mistaken, CSS needs to be written inline for AMP and so I assumed that the spec (based on the author's post) would require that the image be specified in exact pixels.