That's your answer, it's a pretty slow progression so it won't ever reach crazy amounts like other coins, plus mining is still involved.
Does the PoS reward of 10% ever decrease or will it always be 10%?
No it won't decrease as far as I am aware.
Sprouts is 2% interest every day so when you are eligible to stake on the 5th day = 10% stake, if you mint on the 6th day it will be 12%, on the 7th day 14% & so on.