I think a one strike ko was all we could have expected (not that I like it)
Considering that he was so bad-ass him losing out in a proper fight would have caused a lot more of a backlash IMO.
Plus now it leaves the door open for someone to say that he wasn't in full fighting form (maybe his bankai was weaker as well because he wasn't whole) when a weaker guy disposas of juha.
Btw: In the flashback I thought for a second that the painting represented yama's bankai/shikai and that the demon he referred to was what he sealed to create. I was like yeh damn now that he's dead let it reappear...and then my bubble burst!!