Ummmm...you do realize that the name of the show was
Life on Mars. It makes absolute sense. I think the ending was unexpected because no one thought it would be such a literal translation of the show's title. And when I say 'no one', I mean 'me'.
As far as your other comments, well...it's a work of fiction, a fantasy. Anything can happen. Even 65 year old astronauts.
Anywho, I loved the show and was so disappointed when it was cancelled. Thank goodness for DVRs.