Whoa, there, pard. The quality start, 6 innings, 3 or fewer earned runs, was created by a sportswriter in 1985. It's not an official stat and can be used or not. Like WAR and other stats. What has helped the quality start stat to stay alive is that you can also show a stat on the percentage of starts that are quality starts. Kershaw, for example, gets 71%, Tom Seaver 70.2% Price 68%, Bob Gibson 68%, Randy Johnson 67%.
This season so far: Verlander has 14/17 quality starts, Greinke 13/17, Scherzer 13/16, Ryu 13/15, and Berrios 13/16. Chris Sale is 9/16, Porcello 8/16, Price 6/13, and ERod 6/16.
The stats guys are telling front offices and managers that pitchers get hit harder and harder as the number of times they go thru the lineup increases. By the third and fourth time, most of them are at risk of a big rally, especially if their command starts to crumble. So 6 innings kind of makes sense.
Thus too the value of a great bullpen--like the Yankees currently have. Their ace, Tanaka, is 9/16.