What do we do if SETI is successful?

Time is a factor here. How close in time and space would be them?

If we get something coming from more than 100 light years away we might not have the technology to respond, and if we do it may not matter anyway if we are at risk of not having a technological civilization anymore 100-200 years forward. So the meaningful actions on those cases may not include answering back.

Then it will be the actual use of that message. Lets assume that we will decide that is a signal from a civilization that is out there. It will be a signal meant for us and for any other civilization that doesn't have the knowledge/culture level as them, meant for giving us a common ground for communicating back, or it will be something that just will tell us that someone intelligent is out there, but no mean to understand it?

So the options are that we find apparently benevolent aliens willing to contact us, or that we find out that someone is out there but no way to communicate/reach them. I think the second scenario is the most probable one, and how our civilization will react if widely enough will change with time, novelty at first and indifference a few years later.

I cannot imagine any scenario where we're just 100-200 years away from "no more tech" that isn't purely total nuclear destruction. Even then, we'd probably be so close to getting back to a technological civilization that it'd be a blip in the radar at best if we're talking about a society that far away.

We lost 150 years of progress? That's okay, we had 800 more years to advance before the aliens showed up or whatever.

It's such a weird thing I see so many people assuming. We were down to like 16,000 humans on Earth at one point, and that was before we'd developed things that you could theoretically scavenge and jumpstart your tech.

People need to stop doomscrolling; I'm certain this is depression projected.

In Carl Sagan's Cosmos, he talks about how many advanced civilizations could be out there capable of radio astronomy, and how as in our own experience, we have the capability to wipe out own civilization, so that would also be a factor in other advanced civilizations and could act as a limiting factor. There are many such factors other than nuclear destruction that could impact all functioning of an advanced society, rendering it nonviable.

The idea has nothing to do with "doom scrolling". Go watch some Cosmos...

https://www.youtube.com/watch?v=qsl9f83P0Ys