I see Steven lasting as long as Adventure Time will.
Maybe even slightly longer. I don't know why Neoplan Dan thinks they won't do a sixth season. All they said was is that they don't whether or not they'll be a sixth season. But considering the show's ratings here and in foreign countries (#1 Kids Show in France) and the merchandise sales even CN wouldn't be that stupid to cancel the show.
As for Gumball, Adventure Time did well without Pendleton Ward whose to say Gumball won't be different?