Technically yes but at the same time a scarcity of STEEM would increase the market value and still allow you to earn from less STEEM. That and the other tokens that you can now get through the tags.

Your theory is true though as there is only so much STEEM each day and that is divided between all of the vested shares. More people with vested STEEM, less for each person. It's more to do with steem power than people though.