Should show up next turn. It probably hasn't shown up as dead because it takes a turn to send a message, and another turn for it to be accepted.
I used to think just like you, until in one game I found myself being the only player who didn't have more than one type of colonization tech
Yes, trading it does create competition, but people are generally don't establish colonies in your systems because they don't want to start any wars early on in the game. So the only competition you have is over systems that aren't yours in the first place
Weapons platforms - I use them against AI if I don't have mines yet. While there's a good chance that AI will attack your planets, the exact opposite seems to be true with people. Even human players that are psychotic berserkers don't want wars so early on in the game.