Jack Liebowitz had an interest in both National/DC and All-American--which is kind of why they were allied. Jack Liebowitz had a stake in the Independent News Company which distributed the A-A and DC lines. Later in the '50s, Liebowitz through Independent News bought the distribution rights for Martin Goodman's publications, MAD magazine and PLAYBOY magazine.
Something I read in relation to Bob Rozakis's book got me thinking. Either Rozakis or someone else said that DC created a lot of characters in their books as a hedge against the possibility that All-American would break away and take their heroes with them. If you think about this then Johnny Quick seems the most obvious DC counterpart. But I think there were several others--even though they don't seem exactly the same--Green Arrow might have had his name as a counterpart to Green Lantern.
When DC bought out A-A, I think they were prejudiced against the All-American line and that's why most of them got cancelled first, before the DC heroes met the same fate. Green Arrow, Robotman, Johnny Quick, Shining Knight, Congo Bill, Vigilante, Zatara, Aquaman all managed to stay on a lot longer than the A-A superheroes. Jack Schiff and Mort Weisinger were like the senior editors, while Julius Schwartz and Bob Kanigher--having come from A-A--were treated more like junior editors.
Had the shoe been on the other foot, then Green Lantern, Flash, Hawkman, Black Canary, Black Pirate, Ghost Patrol might have stuck around a bit longer. And maybe a few would have even survived into the late '50s like Green Arrow, Aquaman and Congo Bill.