As I said, I wouldn't even have the phone call conversation at all to establish that Khan is dangerous. That was already done with him taking on a squad of Klingons without a scratch. Apparently to these writers, it's not enough to show that he's a badass, they have to have Old Spock say "oh yes, he's legit!"
The thing about Old Prime saying Khan was "the most dangerous" antagonist is that its unintentionally saying "after you deal with Khan, you'll never encounter an enemy as dangerous!". Really though, I kinda hope there is no villain in the next film. That would actually be refreshing for a Star Trek film. Tons of great Trek stories were told without the need to include some villain. THE VOYAGE HOME was the biggest success of the older films, and that not only had no conventional villain but the only phaser used was to lock a door. That's much more ballsy to pull off in a blockbuster, especially today.