Oops, my bad, sorry for that, shippuden's fault! thx for fast reaction on this one.

Anyway, all the seal thing makes me think that it can be weakened only to some point and when it reaches it, it won't weaken anymore.

As I said before, I honestly doubt that it can just break by itself after some time, there's a pressure put on the seal by constant Kyuubi chakra activation and training so it's only natural for it to bend.

I wonder what would happen if there was no kakashi/jiraiya/yamato/opponent to stop naruto in one way or another from kyuubi state.

Ofc, why would Naruto go Kyuubi without a reason? but what if he meets an opponent that he just defeats and there is no ally to prevent going more kyuubi? Would seal make him stay 4-tails eventually knocking him uncionscious or would he slowly deteriorate and grow more tails up to the 9th?