> My main objection is always readability.

Strange, that's my objection to type declarations.

I'm nor sure I would use the word "readability" there. I'd prefer saying that types aid in documentation.

Factory<MarketDataService<Stock>> marketDataService = new Factory<MarketDataService<Stock>>()

Is objectively less readable than

def marketDataService = new Factory<MarketDataService<Stock>>()

And even the latter is not the best.

def MyClass(marketDataService){

    this.marketDataService = marketDataService

}

It's imho the most readable. If the name is self describing and there are unit tests that prove that I'm receiving the correct type I cannot care less about the type itself.

Alternatively, in MLs (this sample specifically is OCaml) :

    type expression =
    | Number of float
    | Add of expression * expression
    | Subtract of expression * expression
    | Multiply of expression * expression
    | Divide of expression * expression

    let rec eval expr = 
        match expr with
        | Number n -> n
        | Add (expr1, expr2) -> (eval expr1) +. (eval expr2)
        | Subtract (expr1, expr2) -> (eval expr1) -. (eval expr2)
        | Multiply (expr1, expr2) -> (eval expr1) *. (eval expr2)
        | Divide (expr1, expr2) -> (eval expr1) /. (eval expr2)

Hey look, I just built a calculator DSL. And everything is statically typed and typechecked (and if I left off the "Divide" case in my "eval" method, it would warn me).