A fab is $5billion. I know of a large company (I'm not allowed to say who, but you might be able to guess) that when faced with some 16bit CPU going out of production considered building a fab - manufacturing is the core competency, so why shouldn't they make our own chips, and thus keep those old designs based in obsolete CPUs that still work just find running for longer. The cost of a fab was high enough they decided not to. Of course on hindsight if they had built a fab it would have opened just as the supply chain problems started, and the company would have made a mint.

I don't think GM will open a fab on their own. Too expensive for what they need. However a joint GM/Ford/Toyota venture seems possible (if it can get past anti-trust laws!), and that would find customers in the likes of Honda.