I too am vey sad The Unit has ended, but I think it was too be expected. It was never in the top 10. It also deals with war which is a very sensitive subject, more so in this climate. Personally I thought the acting was superb and the scripts were very powerful, especially the stories about the wives.
I doubt any petition will save this. People want to make reality shows as they are cheap, too many people are willing to watch them. Shows like Greys and House are more or less in the studio, whilst The Unit had to shoot on various locations. Unless you have a show like CSI which has a very strong fan base the show was never going to survive long.
Goodbye Unit you were amazing!