I know that the Kronos Quartet do not use a conductor, and a lot of "modern classical" music doesn't use one. For example, I don't think I've ever seen a conductor at a performance of a Steve Reich piece. However, such examples involve a pre-planned performance without a conductor.
I'm aware of what a conductor does but I'm interested in what would happen if the conductor disappeared (after rehearsals) and could not be replaced.
How would removing the conductor post-rehearsal audibly affect a professional performance?
Would it just be a bit looser, or would it just become impossible for the concert to go ahead? Would it be easier for "standards" that the players would likely know better?