I don't think you have to go with 100%, you can simply calculate the odds or get the odds from betting sites. That includes many factors, and even the best team can always lose, so there's never going to be 100%, ever.
Yes, as you say, the problem with the probability we assign is that it is always subjective. If 100% were somehow an objective measure you could bet your entire wealth on the bet. For someone with experience and a history of good sports betting results, his predictions have some subjective component but are closer to objective, so if he is paid 1.6 to 1 for a team to win but he is 85% sure that they will win, it will be profitable for him.
But the fact that sport has a human component, and you don't know if the top scorer is going to get injured at the beginning of the game or things like that, makes giving 100% not seem rational.