That's the point that I would make as well. What can you do with an
object that is only known to be Subscriptable?

def do_subscriptable_things(obj):
    if isinstance(obj, Subscriptable):
        # Now what?

Maybe if you want to use/abuse it as an alternative function calling syntax-- square brackets rather than parentheses.

The language itself already contains one example of this: generic typing syntax.