Number of stars doesn't matter if a player is getting 5 form or less. He is useless on the pitch, pain and simple.
What isn't very obvious to the novice is that a 1* player getting 6 form perform better than a 9* who got a 5.

Bad form players ruin your team.
And it is not exactly easy to fix a player with bad form unless you have hundreds of green packs.


In the end, it's still pay to win as you can fix a player with bad form or even keep constant 9 form by powertraining no matter what age the player is.
But, it is lots of cash sank and lots of work (as training has cooldown you can't do it whatever you like), perhaps only a handful of people do it.