As more people join this ecosystem, the amount of STEEM in the reward pool gets diluted. That means it is harder to earn the same amount of STEEM. This will be offset by the idea that as STEEM gets "scarcer", the price should increase.
This might well be a more pronounced effect than many realize. The amount of Steem being added to the ecosystem is less than 9% per year. Signups are a bit harder to measure since the manual system varies so much from day to day but I did some rough calculations that estimated the creation of new accounts here at ballparkish 10-11% per month.
I would say those numbers, rough or not, give a good indication. Even if your numbers are off a few percent, the fact that you are looking at 120%-130%+ growth per year in users while the token inflates at 9%, that means there is a lot of dilution.