>code split into two files and three functions
You could always put multiple classes in a file if you wanted to organize that way. The new language changes just reduce nesting and boilerplate the IDE already took care of. (which isn't nothing). These framework changes to focus on lambdas is pretty nice though.
I'll still be using traditional controllers for my endpoints, however the syntactic sugar in C# 10 does improve the experience by reducing nesting and noise in general (file-scoped namespaces & implicit global usings).
Overall I'm pretty happy with it.