It's not the upper bound that matters but the frequency. How frequently should the compiler assume an 's' appears in the dataset, or any other character?

We know that E[# of '\0' in a string] == 1.

But what is E[# of 's' in a string]? Is it greater or less than E[# of '\0' in a string], and how should the compiler know this?

You haven't given the compiler any reason to assume that 's' or 'p' will appear more often than '\0'.