right now, further rift levels give you more star rewards, but in the future i may only give out star rewards the first time a boss is defeated. i want players to be able to go back a level and farm rewards, but you're right that there should be more incentive to progress!
yeah, i think you're totally right that the incentives are a little shaky overall. since each level is harder and gives the player less kills, it will naturally mean a big drop in income as soon as you enter the next level. i'm going to prototype some new strategies to see if there's an elegant solution!