"If the company does decide to run OpenStack, it's likely they won't contribute back any source code. (Same as Linux situation where most users of it don't contribute source code.) This removes another subset of potential contributors."

From my perspective, companies that decide to run Openstack do contribute code back, but the process is so long and arduous to get code accepted that it's not hard to see how groups would just give up and just merge upstream to their own.

Core reviewers have to accept your code, and they are on no timeline to do so. Months will go by without a review.