Isn’t this just a case of not wanting to or having the resources to maintain a fallback renderer?

Supporting basically-compliant Vulkan GPUs covers probably 99.9% of developer machines. You need a pretty big user base to make it make sense to support the last 0.1% of people without access to a proper GPU driver.

I hope Zed reaches that scale, since I really like the project. But right now I would be a bit disappointed if that was a priority.