You haven't been to Banbury on the M40 then, one of the sites Tesla are opening. 16 bays of Instavolt are nearest to the Motorway, with a Costa next to them. Tesla are further away in a retail park, where there is also a 6 bay Osprey site in the same car park.Instavolt are certainly a good network but they are mainly 2 chargers in KFC carparks at present not 12 on the motorway. I have never been in a situation where it was more convenient to go to an instavolt on a road trip than use the SuC network. They don't have a lot of banks of 8-16 chargers on major highways which is where a lot of people want to do their fast charging. both for location and for the security of knowing there will be working chargers and probably less congestion. So if you think opened up superchargers on motorways would not attract a lot of non Tesla's irrespective of price I have to say I think you are mistaken. And until there are a lot more similar options I don't think that will change.
Or Greys/Thurrock SuperCharger that's just round the corner from the 12 x 350 KW Gridserve Chargers.