Well, a good way to examine your argument "a player with allstar potential will always be a bad player" is to go to the Transfer list, filter out players with allstar potential or lower, sort them by TSP or salary, and see what you get. Currently the best has 122 TSP, and I know some of them even starts regularly in a tier I league. I don't know what is "bad" in your perspective, but clearly those players are good enough to me.
Looking into the game manual, star rating is the one which showed up after being scouted once, which is referred as "a more accurate assessment". That said, it is still an estimation.
Therefore, when you say "star rating max 4 balls when potential is 3 balls", do you also acknowledge that star rating is still an estimation, but you feel that it should be more accurate?
I just want to try to anticipate the implications if what you said is implemented. Now most people will only interview players with five balls star rating, and it takes less scouting points to find a 18 yo, 9+p player. As a result this might make tanking a stronger strategy than it currently is. And I won't like that...
Of course things might be more complicated. Maybe this will also stimulate a lower pick team to invest more scouting points... But clearly it would change drastically in scouting strategies.
Currently players with 5 balls star rating could have only 2 balls potential, so your suggestion could change a lot. Mind elaborating a bit on your rationale?