I would think/hope that any discussion around sharing the supercharger network with another manufacturer would also include a discussion around the other manufacturer also contributing. Having other vehicles charging at Tesla stations doesn't cause any negative impact if the same number of Teslas are charging at non-Tesla stations.
This should be easy to do. Make a contract where for X vehicles you sell with Supercharger(tm) plug, you have to either pay Tesla, or install your own chargers. EZ, and everybody wins.