This thread has gotten a lot of attention. Instead of bringing back autoplay, one idea would be that when a playlist is done playing, to jump immediately to the "Recommended Songs" section. The way Spotify worked a few weeks ago, it would autoplay from Song Radio for that individual song when it was done playing.
I imagine that the reason auto-Song-Radio was disabled is that it maybe didn't fit with user expectations. You pick a song out, listen to it, and then all of a sudden you hear something else. In mobile, the priority is to keep you hands free, so that makes sense in that context.
But it's such a useful feature for discovery, that maybe it just needs to be tuned right for desktop.
(I don't know how to submit to the New Ideas queue, so I've posted here)