Hey, thanks for sharing this feedback!
From your description, I assume you are playing against ai?
Missile frigates are indeed very powerful ships, as they have high pierce, which negates durability of almost any ship. This makes them specialists at killing capital ships as well.
However missile frigates, including Kanraks are not the absolute answer to the game. In fact Kanraks would lose both to Ravastras (light frigate) and Defensor Corvettes. However both of those ships, which counter Kanraks, would fare worse against capital ships.
Point defense balance is a very delicate matter, and we are constantly improving on it, as we address such concern every patch. If my assumption about ai opponent is correct, it's more of a matter to make AI more resilient to those kinds of strategies. If I may ask, what difficulty were you playing on?