Save a sandy streusel

As a general rule, when I make streusel topping, I never use soft butter...always melted. That way, when it mixes with the dry, it forms little lumps of topping instead of a sandy mixture (I suspect you didn't let it mix long enough to start sticking together, and thus it won't bake up like a normal streusel).

Add milk or water SLOWLY by the spoonful and mix with your hand until you have the desired texture.

I'm probably about 6 hours too late, but I'd melt some butter and drizzle it over the top and then bake for a little longer.