Actually the issue you're complaining about is not a Haskell problem, it's entirely a library problem. It so happens that the standard library has a really poorly defined numeric hierarchy at the moment. If you want to fix it right now, you can avoid importing the prelude and import a different numeric library which has more sensible definitions for everything.

The issue is that for most things you'd want (+) to be defined like: (+) :: a -> a -> a

But actually, to be more flexible it could be defined as: (+) :: a -> b -> a

So then all the normal stuff would supported, as now, but you could also support things like:

    instance BetterNum DateTime where
      (+) :: DateTime -> Duration -> DateTime

And so on.