Remember when I mentioned that I suspect there's a quiet battle in the Tesla board about whether to remain an exclusive luxury automaker or go mainstream and compete with Ford, GM, Toyota, etc? The mainstream push is being led by Elon Musk. His overarching goal is to make EV's affordable and commonplace, and he's willing to opensource the Tesla charging connector and the Supercharger network to get there. So an order to open up the Superchargers to other manufacturers would come directly from him. It's not particularly a good business decision, but it fits with his personal mission.
But there's still that battle going on at Tesla, and there is a pretty good chance that without Elon breathing down their necks, some of the board members will find some way to sabotage the deal and keep the Supercharger network proprietary and closed.
I don't see how opening up the SC network is going mainstream. If Elon's "personal mission" is spreading EVs (which I don't believe is his ulterior motive), then ensuring Tesla's continued success should be his only goal. There's still A LOT of growth happening at Tesla. The new S/X aren't fully caught up, the 3/Y are going further and further backlogged, and then there's the yet to be released CyberTruck/Semi, not to mention the still undefined "$25k" Tesla. There's just far too many unknowns to open up the SC network to non-Tesla vehicles.
If Tesla didn't exist, every ICE manufacturer would only have EVs on a drawing board somewhere in a basement, and they'd be perfectly content with going back to that.