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

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.