Since everything in physics tells us that FTL travel is impossible (as far as I am aware), then when a civilization detects signs from another civilization in another solar system it knows that 1) that civilization was advanced enough to produce signals into space light years ago, and 2) by the time it reaches it at sublight speeds (hundreds, possibly thousands of years) it will be more advanced still. That means your extermination mission might be facing a technologically superior enemy with home field advantage.
This may be where intuition and cosmic scale don’t align. The Milky Way is very old and is ~100k light years in diameter.
Say you are a few million year old civilization and you detect radio from a civilization 100k light years away takes 500k years for you fleeting arrive. You are still millions of years ahead in technology. If you are a stagnant civilization you probably stay home cause the other side would overtake you, but then again that motivation to get to them as early as possible. In any event presumably as your fleet get closer you would have a better idea of the technology difference and can just do a U turn
Civilizations that come into contact will tend to be those that are closer together, particularly if they make any effort to be inconspicuous.
The longer they go without contact, the wider their reach, and the greater their odds of contact.
We might suppose that the technology necessary to obliterate another civilization is arrived at early, and is hard to counter despite long subsequent development. Alternatively, civilizations that succeed in suppressing weaker ones eventually meet their match.
Unless you assume each solar system is a threat, so you proactively send automated machines to each solar system that can adapt to the threat as needed. They might also not care if there is or isn't life there and just want to build Dyson spheres everywhere.
The argument for Dyson spheres is extremely weak. If you are powerful enough to build one, you are also powerful enough to dismantle a nearby gas giant and fuse that.
The energy required to dismantle and fuse a gas giant would be tremendous - there's definitely an opportunity-cost involved.
Compared to a Dyson sphere - which only requires a tiny, tiny, tiny fraction of the energy absorbed to build and launch the orbiting platforms that make-up the sphere.
You can "ignite" a gas-giant by maneuvering a black-hole into position inside the gas-giant, but (as far as we know) there are far more gas-giants than there are black-holes, and the distances involved are tremendous (nearest black hole: 3,000ly, nearest gas-giant: 6 to 9e-5 light-years). It just doesn't strike me as feasible or even worthwhile even if it was feasible.
Maybe as the heat-death of the universe approaches, the last band of remaining humans (don't ask) try to ignite the gas-giants to keep the lights on?
There is no need to "ignite" the gas giant. You just skim off hydrogen and fuse it in magnetic confinement chambers anywhere you need power. Gravitational confinement is for amateurs.
We don't build bonfires in our cars, as used to be done in steam locomotives; we oxidize carbon differently, and generate a pressure differential by entirely different means. Why should advanced aliens do things the clumsy way?
Anyway there isn't enough material in a solar system to build one. If you can get elsewhere to gather the stuff, why bother to bring it back?
Which is why the expected strategy is to seed all systems with autonomous berserkers. ... and/or have them meandering from star to star to report/destroy civilizations in their infancy upon detection of radio signals.
You'd need trillions of them per galaxy, but that's well within the capability of a type III civ.
It reminds me of the extra solar object detected a couple years ago that buzzed the Earth and then mysteriously ACCELERATED out of the system.
So assuming we were just detected and assuming there's a berserker in the system already (likely in an outer orbit) that would imply we have only a couple decades left.
This is explored in the novel The Forever War. Since the war had multiple battlefront (with vast distances between them) an attacker never knew if the particular branch of enemy was already ahead with their technology.
