No, Rondo would not make us contender alone ( with current roster and coach we have) if traded for Kanter right now or next year. But we would become instant playoff team for years and adding somebody like Lamb in draft could make us contenders in 3-4 years. But keeping Kanter ( or anybody on Jazz roster) and hoping to draft or get some free agent PG in Rondo category does not make us contenders either. To much unknown, could be good or could be not.
Rondo is proven All-Star, plays through injuries and has championship experience and is still young and in his prime for 5-7 years at least. That is great window to become contenders if we keep Favors, Burks and add some serious scoring threat at SF like Lamb ( Barnes would be nice to but that is a stretch).
Plus think this way, does this team lacks rebounding and post up play or it badly needs passing, leadership from PG position and defense? Did we lose more when we get kill by bigs or by opposing team PG's?
Your thinking is similar to this - you have a chance to cash in 100.000$ winning lottery ticket and make you life better instantly but you would rather invest it in more lottery tickets in hope of winning 10mil in the future.... odds are you will end up with nothing.