A common mistake that people make when trying to design something completely foolproof is to underestimate the ingenuity of complete fools.
Before Sasuke can confront Naruto, he first needs to defeat Obito and if Naruto were to die fighting Obito, well the winners written the history, so technically Sasuke would have proved that he is right, kukukuku.
Sasuke next progression lies in obtain Madara's will. i beleive madara will is enough to progress him closer to Naruto level. He doesn't have time for Rinnegan. Also his fight with naruto will decide who take Hokage's seat. Kakashi can play an important role such discovering what lies within Obito's dimension. For instance, few Uchiha survivors or Sasuke parent. Death of the Uchiha clan doesn't add up unless Madara was the culprit. Obito is weak offensivly and Itachi was fighting time; two Uchiha destroying an entire clan seem a little far-fetch. Anyone but Madara, not an icon possessing Madara will.
In regards to Hiruzen, look at Madara. Look at the Madara that met Obito, and then look at the Madara in his prime. There is a major difference in power. I believe it is the same story with Hiruzen. Hiruzen was never on Hashirama's level, but because of his jutsu mastery, he got the same name tag. His stats in the data book are also very impressive. He has a 5 in Ninjutsu, Taijutsu, Genjutsu, Hand Seals, and Intelligence.
We have yet to see the professor who mastered all the jutsu of Konoha. And even though we may never see it, remember that the ninja in the story of Naruto were far stronger in their prime, than they were in their later years. I do believe he truly earned the title of Hokage.
There was a point where little was known about Hashirama, and that persisted for a great many years. The same goes for Minato. It was all talk until we finally saw it for ourselves on the day the Kyuubi attacked Konoha (Even though I believed it lol The same goes for Hashirama and Madara. There was a guy with the right theory on them 5 years before they were reintroduced.). We're just now learning how great Tobirama truly was. Prior to the year 2013, all we really knew was that he was the greatest water style user of all time.
So even if Hiruzen is never shown again, I wouldn't underestimate him as shinobi worthy of the title Kage. I hope you guys are right in saying that Kishimoto wouldn't bother bringing him back without purpose. Perhaps he'll play a role when Orochimaru and Co. arrive.
I believe Hiruzen in his prime is around the level of Jiraiya/Orochimaru (with less power than Jiraiya, less forbidden jutsu than Oro... in other words, an "All-Around" type, similar to Kakashi, but with more chakra).