he would barely have the power to seal anyone above jounin level though, he cant beat Orochimaru after all.
I'm guessing if he knew it he wouldnt be able to pull the enemies souls out completely because he wouldnt be strong enough.

as for all fights went down hill since Zabuza/Haku what are you talking about?
did you miss the whole Orochimaru vs 3rd sacraficing his life to save his entire village?

Zabuza/Haku fight had no real meaning behind other then to complete a mission to stop Zanza, Haku jumped in the way of Kakashi's chidori but who wouldnt save their friends in a situation like that? nothing deep about that at all. the 3rd sacraficed himself to save not only everyone in the village but future generation of kohona people - its just we havent seen it animated yet so people relate back to the music from Zabuza/Haku.