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 the

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 https://en.wikipedia.org/wiki/Logistic_function and https://en.wikipedia.org/wiki/Gompertz_function https://en.wikipedia.org/wiki/Gompertz_function , which you can imagine as being at least as probable as an exponential rate.