Screen and stage ...
It would take a whole essay to discuss this matter, and I cannot invest that kind of time right now.
In short, though, I would agree with your professor. Unfortunately, all the nuance is missing from what he said. Your report is probably completely accurate, but the professor most likely did not say everything that he really felt.
First, all statements are generalities, not absolute truths.
Film is about producing a product. Scripts generally get rewritten and rewritten, usually with more than one writer. Film is about product. Some personal vision was probably in the mix originally, but it also probably did not survive the process.
Stage is an entirely different matter. That is where the individual artist can create his own vision. Will some changes be necessary? Probably, but they are in the hands of the author. Your play is not going to become the property of someone else who will proceed to change it.
My real question would be: If you are a writer, why cannot you write your own play? If you cannot do that, then you are just interested in product, a show. Nothing wrong with that, if it is what you want to do, but it is not art. Consensus always kills artistic vision.
I think that what Doug mentioned is a different situation from the kind of collaboration that I am talking about (and I am obviously not too keen on collaboration for the stage). If you take a collection of personal experiences and turn them into something with integrity, with a thematic vision, you are still the artist who is molding those experiences.
As I said, it would take a whole essay. I feel that, like your professor, I have given too absolute an answer, but it is the best that I can do right off the top of my head.