In addition to umanwizard's comment, you're assuming that there are no external constraints involved. There are plenty of non-exponential growth curves out there, like https://en.wikipedia.org/wiki/Logistic_function and https://en.wikipedia.org/wiki/Gompertz_function , which you can imagine as being at least as probable as an exponential rate.

That assumes the marginal difficulty of further improvement stays constant as intelligence increases.