In addition to what's been said, I've heard that once the pseudo-pop to prominent stamina happens, you'll drop back to proficient the next week guaranteed. A true prominent in stamina doesn't exist, there's a hard cap at 10.0.
A player would play way too much with prominent stamina anyways. Probably the full 48 even if he had a back up.
You are absolutely right, and it is a shame it works this way.
Stamina and substitution patterns are two of this game's most unrealistic features. They seriously detract from the authenticity and tactical options of real basketball.