Looks like you are assuming the market cap for Steem remains the same. If that is the case then you are correct. If people are producing content that adds value to the platform, the market cap will increase; 8% is not exactly a huge annual increase to compete with. Take a look at Facebook for example, they've had many years of more than 100% increase in market cap.
We could argue 8% inflation per year could be too small. We need a rewards pool large enough to attract content creators with sufficient talent to attract more attention to Steem. The more activity we have on the platform, the faster the platform will grow. Eventually, we should see post sponsorship as well as advertisements appearing in the posts that are attracting the most attention.
In short, there is plenty of incentive to hold Steem even if our Steem holdings increase by less than 8%.