I am hoping they will be leaving dealerships behind entirely. From what I've seen in the videos, this thing is SUPER simple, and that bodes well for service, which is half the battle, the other half is parts.
For Parts, they've talked about having an online catalogue with exploded diagrams and free workshop manuals, and the parts are slated to be available to order from their website (along with the car) delivered right to your door. Or, to the door of your local mechanic.
For service, that local mechanic would be key. Every small town in the world has a ragtag shop with a Kawasaki, New Holland, John Deere, and Stihl logo on the side of it. They fix everything in these shops from chainsaws to farm tractors. And, there's no need for XYZ population density to support the small business. I'd love it if I saw the Ineos logo along side all those other ones at these small-town dealerships.
Isn't BMW the car company that actively sued British small business owners (mechanic shops) who had Mini or Land Rover logos on their shops? That doesn't bode well for a "get it fixed anywhere" approach. I have not seen any confirmed statements on this from Ineos, but the "vibe" I've been getting from Ineos would make "get it fixed anywhere" fit a lot better with the brand/intention than "get it fixed at a limited dealer network". Further to that point, outside the major cities in North America, BMW dealers are few and far between. As in, there's 500-1000 kilometres between BMW dealers here in Canada. I don't know what it's like in the states but anecdotally they are not nearly as common as Ford, GM, Chrysler.
That being said perhaps this will be a 'both, and' situation -- no reason (if those involved want to play ball) that they can't partner with BMW for service and parts AND the shade-tree "every town" mechanics as well.