> See how fast technology is developing, and especially the decreases in the cost of rocket launches.

The past century of rapid progress is anomalous in human history. We made do with stone tools for 100k years. In the middle ages basically nothing technological happened for a 1000 years. Consequently, this view of eternally ongoing progress is an artifact of the specific time and place we find ourselves in and not something that really has to happen. While we had some good results from semi-conductors I see our progress as already slowing down. Most of our science doesn't reproduce. Our cosmology is in shambles (more on this later) and our engineering is surfacing issues around putting the correct number of bolts on airplanes.

> Within the next few hundred years humanity (or our drones) will have spread to countless habitats around the solar system.

Why? What is there beyond the gravity well that isn't on earth? Maybe there are some substances, such as deuterium, that we can exploit remotely but this idea of "space habitats" is just romantic science fiction. Maybe someone will make it happen but only because we humans are in love with space, not because it makes rational sense.

> Within a few thousand years, we will have started building a Dyson swarm that will obscure a measurable fraction of the light of our sun.

Again, why? Fusion and fission can supply all the energy we want. If we want to do mega engineering for more energy we could just bore down to the earth's core. I'd bet that's what intelligent aliens on Enceladus would do. Regardless, it's completely useless to predict the future a thousand years out. We simply don't know and we have no data to anchor our fantasies to reality. From this initial starting point, nobody ever made and predictions worth anything.

> We don't need any new science for this, nor radically new engineering.

So why don't we do it then?

> For this confident prediction, we only need a few key ingredients:
>
> * Humanity doesn't blow herself up completely. (Civilisations blowing
> themselves up is one way to resolve the Fermi paradox.)

Like I showed, you are baking in more and more assumption. Basically every sentence of yours is one more huge assumption.

> Even today's technology could detect the humanity of 3024 from countless light years away: just point a spectroscope at the star and notice a vast excess of infrared (the waste heat of our solar collectors has to go somewhere) and an corresponding deficit of shorter wavelengths.

Some SETI and Fermilab people looked already back in 2006 and found 17 candidates. There is this flawed idea that we already have detected and interpreted everything that there is. But JWST is showing us right now that we don't know anything. Every piece of our cosmology is currently under reexamination because wherever we point JWST we see stuff we can't explain. Even just the question of cosmic distance measurement isn't simple and clean cut.

All our space science is based on the premise that we are doing natural science where natural things exhibit regular patterns that we can describe with laws. If the universe were to contain ubiquitous mega engineering we would, by definition of the science we are doing, not recognize it. For instance, your Dyson spheres detection assumes that we see an abnormal spectrum. But, abnormal compared to what? If the universe was full of Dyson spheres we would say that there are "infrared stars" and then we would hypothesize how these come about naturally until someone can crowbar in an explanation that sort-of-kinda could work. Cosmology is full of these kinds of after-the-fact explanations btw, just look at Tabby's star or Oumuamua.

So again, we end up at a place where the Fermi paradox isn't very impressive. If the aren't building (arguably pointless) mega structures, we won't see them. If they all are doing mega engineering we wouldn't recognize it. For us to see them the aliens must be doing things that are very 20th century western hemisphere human things.