I find myself inclined to the opinion you're disagreeing with in all honesty.
When defining types in other languages, the task is prescriptive (you specify what fields there are in a type and the runtime accepts this as law), but in Typesxript the task is meant to be descriptive (as you say, one models the types that exist at runtime which is the inverse).
I was excited about Typescript when I learned it, but found myself disillusioned by actual experience when using it (of course others love it and have good reason to). Had defined classes in Typescript so I can have some of my types reflected at runtime.
