I mean, if your starting point is, "The enemy lurks everywhere", that kinda is a kneecapper for anyone to convince you otherwise, isn't it? You assume that every new polity is going to be horrid, instead of potential ally or trade partners. You assume that the future holds uncertain doom, instead of hope for a better future.
If you are unable to go past that pessimism, then there's no arguing with your points.
Then again, let them come. Which polity, exactly, holds Legacies that enables them to a) innovate more and b) innovate better? Not the Maradysh. It's the Arthwyd. We can uptech better, we have the unbeatable edge of population. We have a Vassal, loyal to us and will aid us without our prompt. Not the Maradysh. It's us.
Don't be defeatist or conceal them with a "kill them all" approach. This is a set-back, but be the better man and not throw blood away.