There's nothing artificial about the differences between Bluegrass and Old Time music as being played these days by current bands. Peoples tastes in music have become much more focussed in recent years, due to the easy availability of all sorts of music, and the names of the genres should reflect this new focus. 50 years ago it was all classed as 'Country Music' - nowadays that term has been hijacked by rock bands from Nashville with Stetsons and cowboy boots. I wouldn't want bluegrassers to hijack 'Mountain music' as a title.
Cheers
Ray