> We shouldn't force devs on Bower or Webpack. If I want to try out a new theme just bought on ThemeForest, it shouldn't take hours to make it compatible. Or to force to do the /priv/ hack that seems unelegant.

You could at any time have installed Phoenix without those. And what's so hackey about /priv/? That's where you can dump things that you know will be served. It's literally the intended purpose of the folder.