You must be assuming that the price of STEEM stays constant.

If you are not considering STEEM price and only STEEM quantity, then yes, you would need to earn the similar amount of STEEM, however it won't need to be at much as the 8.61%. The reason is that you earn interest when holding SP. It is a small amount though and shaves off about a percent.

Clearly, the market factors dominate price which is king and not the inflation.

In the future this might change. "IF" STEEM blockchain becomes popular for businesses to invest, startups to happen, and SMT's become a reality, then liquid portion of STEEM may become a limited resource. This is a whole different dynamic that most other currencies don't have.