Maybe you already know, Youtube doesn't support anymore the API that smtube was using. I don't feel like working to support the new API. First of all (if I'm not wrong) the new API requieres to register the application with Google, and so it's necessary to abide their terms of use, which of course don't allow to download videos. So a smtube using the new API won't be able to download videos (which I think it's its most interesting feature).
Maybe it would be better to implement a plugin for web browsers to add a "play in smplayer" button in sites supported by mpv/youtube-dl.
And regarding smplayer, since mpv+youtube-dl already can play youtube videos, maybe it would be better to remove the current support for youtube in the smplayer code. Or, if people still want to play youtube videos with mplayer, maybe smplayer should call youtube-dl to get the youtube URLs. This way I wouldn't have to update the "youtube code" twice a week.