If you want to convince people building software enabled businesses to keep their source open and free rather than closed, a better rationale is needed than "I've never seen it happen so you should definitely bet your business on it never happening to you."
The "Uber for X" mentality exists, but as OP admitted, it exists without the source code. Uber is not open-source, nor is Twitter etc. and yet, they still get cloned. So did Reddit, as an idea, but I don't think the availability of that code helped the clones much.
