I have a little app that lets me send it a Spotify playlist and then it goes and finds the matching song on YouTube, converts it, and downloads it all in a big batch. Took forever to fuckin' find and it was of course on a site like that. Been a while since I used it; probably doesn't even work anymore. I tried like 5 others before finding that one, and they were all broken.
The dumb thing is I only wanted it because even though I use the local file thing and still pay for Spotify, it wouldn't actually play anything whenever I didn't have Internet access, making it's own built in system of "downloading" and playing files locally useless.
How reliable is the song matching on YT? I'd imagine some songs would be obscure or exclusive to Spotify so that you can't find it on YT, let alone it being a low quality one.
I'd approach this problem by using Spotify's API (or scraping) to get the good stuff, then sending it to my phone whenever I'm connected to the wifi.
YouTube ends up having more of the obscure stuff I like than Spotify does, actually so for that stuff I can't even add it to a Spotify playlist.
It's remarkably good though. I mean even if you're just putting a little vid up to show something off really quick, it can remove the audio if it picks up a track playing on the radio in the background and is usually correct in identifying the song.
So you managed to initially think of a tool that can automatically do this for YouTube, but not any other platform? come on this is sad, just think for a second.
All I did was search for a tool that could take my Spotify playlist and download mp3s of it. There was no such tool that used any other source besides YouTube. Probably because no other searchable source allows downloads through an API while also comprehensively and correctly labeling the music used.