If a pitcher goes 6 IP and allows 3 ER, thats a "quality start". Also a 4.50 ERA for the game.
If a pitcher goes 8 IP and allows 4 ER, thats not a "quality start". But also the same 4.50 ERA for the game. Doesn't make sense. Depending on the team and their bullpen options, some teams might prefer the 8 IP and 4 ERs to the 6 IP and 3 ER, all things being equal. Like if you have Zach Britton, and 6 Wade Mileys in the bullpen, 8 IP and 4 ER isn't a bad deal. If you have Brach. O'Day, Britton at the top of their game, 6 IP and 3 ERs will do.
Maybe a quality start rule should be 1) at least 6 IP, and 2) 4.00 ERA or better for the outing. (4.50 ERA seems too generous).
That way 6 IP and 2 ER (3.00), 7 IP or 8 IP and 3 ER (3.86 and 3.38 respectively), 9 IP and 4 ER (4.00...I think....) would all be quality starts.