Some rebuttals, going point by point...
1. you can know the intentions of other entities by observing and communicating with them.
2. technology explosions, like pretty much exponential phenomena, are self limiting. They necessarily consume the medium that makes them possible.
3. and 4. civilizations aren't necessarily sentient (ours certainly isn't) and don't have an agency, much less goals. Individuals have goals, and some may work for the survival of the civilization they belong to. But others may decide they can profit if they work with the aliens.
4. Multiple civilizations may well come into competition over resources, but that's more of an argument about why the forest would not be dark.
Practically speaking, a civilizations that opts to focus on massive, vastly expensive efforts to find and exterminate far flung civilizations because they may become a rival in the future may be easily outcompeted by civilizations that learn to communicate with and work with other civilizations they encounter.