I always interpreted less as “they let Robot get away with it because he was right” and more as “they let Robot get away with it because it was too late for any alternative”. His takeover was so quick and comprehensive that removing him from power after the fact would just leave a massive power vacuum that no one else could really fill at the time. The Viltrumites could hypothetically fill that void by sheer force, but Nolan wasn’t interested in going back to their conquering past, even if it would have been (hypothetically) justified in this case. They didn’t go ahead and put him down out of revenge because it just wasn’t worth it, and would come at a high personal cost that many remaining heroes, like Mark and Eve, just weren’t willing to risk. The series as a whole is full of instances of revenge or purely punitive action just not being worth it or helpful, and Robot’s takeover is the epitome of that. What Robot did was monstrous and just wrong, but once it was done there just wasn’t much to be gained from taking him down, and everything to lose.