But what if the Klevans land in the "delta" quadrant and have a big ton of Borg to deal with, or the "gamma" quadrant and have the Dominion. They may not like it here.
Maybe Rojan's decendents are the key to turning back the invasion, as they know the Andromedans but aren't Andromedans and therefore enemies of them.
Actually, Rojan's attitude toward Capt. Kirk didn't seem completely merciless. I could see if the Andromedans showed up and said "You're conquered, behave." and the Federation said yes, that it would be business as normal almost. Somewhat like when the Romans conquered an area and the population was not in revolt.