So I think your argument makes a ton of sense but it depends on the organization and culture. As well as the product or project being designed. I’ve worked in places where building a prototype really helped kick things off. And in others where building a prototype with code wasn’t actually the best approach.
Instead of forcing designers to code I would argue that both design and development need to learn to coexist and collaborate. Meaning design should understand the medium they are designing for and speak a bit of the language. But also developers should learn about design thinking and what actually goes into designing.