That's not the "main" issue. I think you're phrasing this incorrectly. The process of
betting on a sport is speculative (as is an options/stock exchange bet). The outcome, and probability of that outcome, are not known to within 100% confidence; that would be impossible. But, you CAN know, to a satisfactory level of confidence (say 95%), that a result will happen. So yes, the formula's inputs are never TRUELY known, but they are known to a high enough degree of confidence that Kelly is worth using. Indeed, then, you are speculating. But it's not the formula, it's the setting in which it is used that is speculative in nature.
The bottom line is that Kelly WILL maximize your growth (more so than almost any other strategy). It is a matter of how comfortable you are with risk, and what level of said risk you are willing to accept to achieve growth. You may be comfortable flat betting, I may be comfortable betting 1/2 Kelly, a high-level prop trader on the CBOE in Chicago may be comfortable using 9/10, or even full Kelly to achieve a return.
Would be willing to wager that 1/2 Kelly would outperform almost any strategy you can come up with. It is very powerful when you know your edge.