I have been wondering the same, but slightly differently.

Tier 1 suppliers have enough resources in both know-how and manpower that I have been wondering if they could do a platform car. Provide a basic frame that passes crash, provide a basic engine that passes emissions, provide basic safety, etcetera.

Then invite other parties to upgrade components. Package lots of air between components to simplify compatibility.

I suppose the only way to get this going in the real world is a big military contract, but I am wondering if it wouldn't be smart play for everyone involved. It would be deadly for a bunch of traditional automakers, but they can't do anything preventing it.