For even a medium-sized orchestra I think a conductor is required. In the end you need one person who decides the tempo and timings. Without a conductor a musician will have to play the part. Here's an example: https://youtu.be/oJ2ZLr87lLY

Clearly the main violinist takes over the role of conductor, but even then timings are off even in this short piece. And that's a trained orchestra with a well-known piece. A full symphony with a larger orchestra and maybe even a choir (like Beethoven's 9th) would sound terrible without a good conductor.