There are orchestras that have performed conductorless fine. I watched my old bass professor, Barry Lieberman also from Orpheus chamber orchestra, play in a gro

There are orchestras that have performed conductorless fine. I watched my old bass professor, Barry Lieberman also from Orpheus chamber orchestra, play in a group of 12 and they did perfectly fine with American String Project and they all listened to each other to get it done. The fundamental reason that conductorless orchestras over 20 don't occur in practice is that music written for that many tends to be complex enough to warrant a conductor. Similar to how start-up companies probably don't need much regulation and a board of directors while multi-billion dollar enterprises invariably do, elaborate pieces are precisely the domain of symphonies. You're not about to need a conductor to have the London Philharmonic play Pachabel Canon or something, right? And boy would it seem awkward for a quartet to attempt Mozart's Requiem Mass. It's why Metallica with a symphony sounds really, really forced - Master of Puppets is more arranged like you would an étude or chamber concerto, not some symphonic score with tons of movements and five layers of polyrhythms. Conductors are very much like popular music DJs and MCs in that they're supposed to be expert listeners in charge of coordination of many parts that could easily fall apart potentially if the musicians started to have trouble listening to each other in an especially complex piece of music. Stravinsky works come to mind, and even world class symphonies can (and certainly have!) fall apart on pieces like Rite of Spring or any movement from the Firebird Suite. As an example of such leadership, I saw the National Symphony Orchestra play in sync with the music from Fantasia 2000 and the only way the orchestra would have known to STOP was because the conductor could see that the video did not match the piece because the orchestra faced the audience. The discipline it takes to full stop an orchestra also reflects the professionalism too. Those that grew up playing music in groups are quite familiar with stopping constantly and taking a while to slow to a stop every time, but you don't put up with that as pros. But in my defense, I just kept going out of boredom of playing maybe 6 minutes in an hour while we kept screwing with the violins or trumpets for the whole time. If people don't practice, we all suffer.