Ignoring the impact of Steem Dollars for a minute, Steem inflation is fixed at 100% per year. However, 90 percent of that inflation goes to holders of Steem Power. So, real inflation is only 10%.

That inflation is only experienced by Steem Power holders if/when Steem token value grows by less than 10 percent per year. If it grows by more than 10% per year, then they experience no economic loss from inflation (when measured in absolute dollar terms).

Regarding paying future users, this should not be a problem. Either future users never come, in which case the current Steem value (or perhaps some lower value) gets distributed among a limited number of posters, in which case there is plenty of value to go around to incentive continued posting.

Or, the new users do come, in which case each one will want to be as "influential" as possible. They have no influence without Steem Power. They have no Steem Power without Steem. So, more users equal more demand for Steem Power and Steem, which means a higher price, which means there's still plenty of economic value (in dollar terms) to incentive continued posting and curating.

In short, the system self-adjusts. Fewer users equal smaller dollar value distributed among them. More users equals more dollar value distributed among them.