> static types often reduce to a bunch of optionals, forcing you to null check every field

On one end, you write / generate / assume a deserialisator that checks whether incoming data satisfies all required invariants, eg all fields are present. On the other end, you specify a type that has all the required fields in required format.

If deserialisation fails to satisfy type requirements, it produces an error which you can handle by eg falling back to a different type, rejecting operation or re-requesting data.

If deserialisation doesn't fail – hooray, now you don't have to worry about uncertainty.

The important thing here is that uncertainty is contained in a very specific place. It's an uncertainty barrier, if you wish: before it there's raw data, after it it's either an error or valid data.

If you don't have a strict barrier like that – every place in the program has to deal with uncertainty.

So it's not necessarily about dynamic / static. It's about being able to set barriers that narrow down uncertainty, and growing number of assumptions. The good thing about ergonomic typing system is that it allows you to offload these assumptions from your mind by encoding them in the types and let compiler worry about it.

It's basically automatization of assumptions book keeping.