Questions about section "Index Signatures"

This [chapter](https://basarat.gitbook.io/typescript/type-system/index-signatures) talks about:
Using an intersection type to slove combine properties into index signature,but this is not useful.
I have a question, Why not use Union Types(|) to combine this properties?
And, Why TS is not infer "never" type, when you use intersection type(&) to merge two types that cannot possibly intersect.
```
type FieldState = {
value: string
}

type FormState =
{ isValid: boolean }
& { [fieldName: string]: FieldState }
```

Start with the linked "Index Signatures" chapter and inspect the FieldState/FormState intersection example. Verify how the chapter currently explains intersection and union types, then clarify why the proposed union alternative and the inferred result for incompatible intersections behave as they do. Done means the chapter answers both questions clearly.

Written by the indexing model from the issue text.