I think the main reason it was over in two seasons was John Simm, he didn't want to do more of it and thankfully understood that a concept like that couldn't run forever. If creators didn't want to continue, they wouldn't have agreed to do a spin-off. But with the American series, I don't see how there could be one, since Keitel is too old to find a love interest in 80's.

Ashes to Ashes had a decent finale for the first season, but all in all, is not as good as Life on Mars. Keeley Hawes leaves much to be desired and 80's fashion sucks. Plus I don't see where it want to go for the 2nd season, or the finale of the series be as mesmerizing as that of Life on Mars.