When has NPC encounters been based on income earned?
You gain rank by destroying ships of near equal rank or above, it's point based. Once you start to reach the higher ranks, the lower ranks don't count towards your Combat Ranking. You could spend 10 years killing Harmless pilots and never reach Elite status.
Not quite true, you get points for anything up to three levels below your own. Meaning at Deadly rank you still get progress from killing an Expert NPC. Which isn't really that hard to do. Source: http://elite-dangerous.wikia.com/wiki/Combat
I really hope Combat Rank isn't tied to what opponents you get, I like the feeling of getting better and the game getting safer, I don't want my random ecounters to scale up with me. If I want a challenge I will head out to a Haz Res or some "Threat 4" USS'es...