> in general you'd have to carefully limit all uses of "with" for objects with precomputed values

What you’re describing is incidental complexity. It is not a good thing. We can’t constrain the complexity with language constraints, we have to rely on the programmer following some guidance and never ever making a mistake. In large teams and large code-bases that's a problem. And it's an offloaded problem from Microsoft to every software development team on the planet that uses C#.

The incidental complexity for the average C# developer has increased, whereas a better direction of travel would be toward correctness and declarative features. I would prefer it if the csharplang team worked toward that.