I'd love to get 10 great seasons out of this...
Unfortunately, unless it takes a dramatic upswing in quality, that's not going to happen... we'll probably have six or seven. It won't make it as long as Supernatural.
Season three had better ratings than season two, and the season four premiere did better than the season two or season three premieres.
It was mostly the crossovers with The Flash that contributed to the rise last season, but the simple fact is what we perceive to be "quality" has very little impact on the ratings. I could rant all day about shows like iZombie or Constantine deserving better ratings than Gotham or SHIELD, but at the end of the day my opinion of their quality doesn't matter.
The Flash is the top rated show on the CW by far, and Arrow is comfortably ahead of Supernatural and iZombie for second place. Legends of Tomorrow will probably land in between Flash and Arrow.
As long as the ratings hold up and the main cast wants to come back, CW isn't letting this show go. Crossovers with Flash and Legends of Tomorrow will likely hold this show above the average CW show for many years to come.
The Vampire Diaries is doing less than half the ratings it did at its peak and they lost their lead last season, and it still has a chance at getting renewed on CW because shows like Jane the Virgin, The Originals, The 100, Reign, and Crazy ex-Girlfriend are doing way worse.
CW also has a per-episode deal with Netflix for their shows, so any show with a decent following is going to have as many episodes as possible squeezed out of it.
EDIT:
I didn't realize how old your post was when I quoted it.