we discover the universe will definitely expand forever. Now what?

You're right, my principle doesn't work if there's something we believe with absolute certainty.

If I tell you an alien living three million years ago threw either a red or a blue ball into the black hole at the center of the galaxy but destroyed all evidence as to which, is there a fact of the matter as to which color ball it was?

If we later find out that the alien did in fact leave some evidence, and recover that evidence, we'll have an opinion about the color of the ball.