When I'm in Japan, speaking to Japanese people in Japanese, I don't expect to be called "Mr." - why would I expect to be called by a Japanese title when I'm in the US, speaking English with non-Japanese?
Of course, there are cultural considerations - around here you call most folks by their first name. It's different in Japan - but, then, we're not in Japan are we?
As to the meaning of "sensei" - it denotes exactly zero about qualifications or credentials, it's simply the description of a societal role.
In Japan you usually call someone by their societal role as the first choice, last name as the second choice, and first name as the last choice.
But, for most of the people in this discussion, we're not in Japan, we're not Japanese, and we're not speaking Japanese - most of us don't even have the ability to speak Japanese.
Different culture, different place, different language, different customs. It seems kind of silly to me to insist on importing random Japanese customs that really have no purpose outside of the context of their culture.