Right.
You're missing the other side of that coin, even though you use it to make your point with Betelgeuse.
Since we're looking into the past, we have no way of knowing if a star has actually exploded or not. There is, in fact, no guarantee that WR104's Wolf-Rayet has not ALREADY detonated, possibly a VERY long time ago.
If that star explodes today, we won't know for 8 millenia. But if it exploded 8 millenia ago, we'll be finding out any day now.
Of course, this is a lot of discussion for a gamma ray burst that only MIGHT be aimed at us.