I agree with your basic premise @innuendo, but applying this in practice is a little more challenging.

If steem continued inflating at this rate forever then investors are scared off by the inability to meet expectations at some point.

If steem changed it's inflation rate over time, for instance as you say, by beginning to decrease the rate of inflation after 2 years and asymptotically approaching 0 over 100 years, it may not coincide with steem's growth chart.

Leaving it as is for now and trying to change the inflation rate later on down the road will be difficult, it is not easy to reach a hard fork consensus once a crypto grows larger.

I guess the solution would be to have the inflationary function based on growth within the last X amount of time. I don't have a concrete answer on this, but I think basing steem's inflation rate off it's empirical growth is probably the best solution?