What do we do if SETI is successful?

In the end, I kinda... don't care. Look up - there's nothing. There should be at least some alien civilizations trying to make their presence known. There should be some signs somewhere that could be recognized universally as either "stay away" or "come here". It really should be trivial to locate technological civilizations unless you've got some incredibly solid reason as to why EVERY SINGLE ALIEN CIVILIZATION IN THE UNIVERSE acts a certain way. Color me doubtful.

We have billions and billions of data points showing the Universe is empty. We have exactly one (1) data point showing it isn't. And that's us.

Besides, just look at the timeline. The universe has only been cool enough, with enough stable stars, with enough formed planets for potential life to form for a few billion years. Between that and the Drake equation, life alone is likely to be unreasonably uncommon. Life that forms after a planet becomes stable, doesn't have any planet-altering disasters, evolves to complex multicellular forms, evolves some kind of intelligence, becomes social, forms a society, advances technology, and starts exploring the universe...? Why bother? The math doesn't work.

Note: I'm not speaking about any KIND of life existing, I'm speaking about technological civilizations. My belief is that we are essentially the forerunners.

The idea that humanity is the only civilization in the entire universe strikes me as the absolute height of human arrogance.

This is an ad hominem argument. It attacks a position not because it's wrong, but because if you advance it you're a bad person.