You bring up Wally West and all you have to do is look over at the giant mess he is in. If The Flash comics wanted to work like the story was always moving forward (no pun intended), like how manga tends to work, or many other types of stories, then a character like Wally West as Flash would be fine. However, DC Comics don't always work that way. They exist in a shared universe and they reboot and reset things. That's when a character like Wally as Flash becomes more difficult. Essentially, you wind up with two guys trying to fill the same role, and that can cause a huge mess.
An occupation that actually matters in his stories.
A consistent family and notable supporting cast (Tom, Kilowog, Guardians... John Stewart
, etcetera )
An iconic love interest.
Strong relations with other characters in the wider DC Universe like Oliver Queen and Barry Allen.
An iconic villain.
A rogues gallery.
He actually matters in the DC Universe. You could take John Stewart right out of it and it wouldn't make any difference.
This is all just off the top of my head without even trying. I could probably think of a considerably longer list if I tried, but I think the point gets across as is.
A better question would be, "What
does John Stewart have?" He doesn't have much because he wasn't designed to. And yet some of you want this character as your number 1 black hero : /
Whatever John has, Hal Jordan has, had first, and has much more on top of that. So, I don't know if that means Hal Jordan can never be replaced (I never said that), but Hal Jordan was designed to be a main character. In essence, the world was primarily designed to revolve around him. John Stewart exists within that world.