You're saying that AMP doesn't help with slow connections, based on AMP not loading all resources on 2G. But you're missing the data point for non-AMP pages on 2G. Are you sure it's not even worse?
It's hard to believe how bloated many modern websites are, before trying to load them over 2G. We're talking megabytes of data and tens of connections to display what should be a 20kB of text.
AMP puts 100KB of JavaScript in the initial render path, and a bunch of CSS. That doesn't mean that other sites cannot be worse but it means that there's no way to make an AMP page which doesn't require transferring at least that much data.
I notice this fairly regularly in marginal network coverage (subway tunnels/platforms) where AMP pages load no faster than any well-optimized site unless the stars are aligned and you actually get a cache hit.
Thanks for raising this. Anecdotally text in both cases appears to render first which makes sites workable. Images are consistently the laggards. In the AMP case the text content is always reasonable (i.e. formatted well) whereas in the non-AMP case you're beholden to whatever CSS the author applies which if it's lagged can cause re-draws - which are quite jarring.
It's hard to believe how bloated many modern websites are, before trying to load them over 2G. We're talking megabytes of data and tens of connections to display what should be a 20kB of text.