Basically, the Unsullied and Dothraki were a powerful army who destroyed the capital, who just lost thier queen and thier mose powerful asset, Drogon.
It doesn't matter about the rivalry between the other realms. The entire realms surrounded King's Landing and were about to fight the Dothraki and Unsullied. The North, the Vale, the Riverlands, the Reach, the Stormlands, the Wesrlands, Dorne.
Not even the Dothraki and Unsullied could stand up to all of them. Greyworm knew that. He didn't want to die.
Well then it would have been nice if they had stated that peace talks were the point. Instead the topic of electing a king came about in a very, "hey guys, what if we do THIS" kind of way.
And even if peace talks were the point, what was the point of bringing along a condemned prisoner who wasn't allowed to speak (who then made the only - and very insufficient - case for the boy who would be king)?