Brice,

Sorry for the delay!  However, I don't really know what more to 
say.

I'm still against pulling in ffmpeg for everyone, for the reason 
given in my last[0] mail.  Keep in mind:

  $ guix size youtube-dl | tail -n1
  total: 184.0 MiB

  $ guix size youtube-dl ffmpeg | tail -n1
  total: 804.7 MiB

If youtube-dl silently failed or produced worse video without 
ffmpeg, things'd clearly be different.  Same if it disabled 
features at build time if ffmpeg is not found.  I don't think it 
does…

One could argue that those who really don't need or want ffmpeg 
can simply customise their youtube-dl package because Guix is 
awesome.  Both true!  But it's the asymmetry between opt-in

  $ guix install youtube-dl
  $ youtube-dl https://hotvidz.gnu/10h-of-rms-dancing.oggv
  Note: you'll want to install ffmpeg to see more glorious detail
  $ guix install ffmpeg
  …

and opt-out, where the user has to learn about 
substitute-keyword-arguments & write a custom package just to 
avoid pulling in ffmpeg, that bugs me.

That's just me.  If you convince enough people to merge this I 
certainly won't mind!

Kind regards,

T G-R

[0]: https://debbugs.gnu.org/cgi/bugreport.cgi?bug=40060#23