So when a bowler gets a strike, he should take up golf, because it's obvious that he's mastered the game??
What the player has demonstrated is that they have mastered ONE CHARACTER CLASS and is working with a group of players that can intuitively work together as a team. The fact that they survived a massive boss battle means little - because there's another boss battle around the next corner.
HOWEVER, I may also be over or under thinking this, because deities are gods for a reason, and can't be permanently slain. Can you imagine what would happen when that ONE evil dragonelle slips away and tells the evil dragons about the group that killed the Dragon Queen? Okay, the party can take on a minor godling... what about thousands of dragons all at once - all with different breath weapons. And what about the NEUTRAL dragons that are ticked off that the balance of power has been upset? We can also bring in the good dragons as well, because they knew that Demogorgon and Orcus have been fighting over Tiamat's holdings for millennia and they are worse than she was - and now THEY are gaining in power.
Yeah, in my world, that would be a death sentence.