With the Builder pattern, you can easily define complex relationships for your class inner state. Like maybe if A & B are specified, then C shouldn't be specified. But if C is specified, then D should have a default value. Etc.

These types of initialization requirements are not easily duplicated with C# init-only construction. Instead, in C# you have to rely on the callers to know exactly how to correctly instantiate your class, including all the complex logic like in the example above.

So yes, C# init-only setters are kind of the best option you have. You can still create a C# FooBuilder for a Foo class. But generally C# programmers want to go out of their way to directly instantiate a Foo, and the number of bugs you get from this is crazy. It's a culture thing, honestly, not as much a language problem.

My two cents.