If you had to opportunity to meet O Sensei today how would you address him??? Nice to meet you Mr. Morihei? Hows it goin Ueshiba?
The title of Sensei has its' place and should be used and applied with common sense to show respect to someone who teaches you. If I run into my instructor in town I don't address him with "hey Dom" I simply say good afternoon Sensei, no big deal, just basic stuff.
PS: As an aside, I have seen titles abused and that says more about the person who abuses it than the ones who address them.