A lot of good shows are starting to be smart and call it quits before starting to run on fumes. Breaking Bad is a prime example where it will call it quits after 5 years, and that show is strong all the way through IMHO. Some other shows I liked such as House carried on for 7+ years, and I was almost waiting for that one to end. Dexter is about to start Season 7, and that is also starting to get repetitive minusthe huge endings to the even number seasons.
I think 13 episodes is too short for a season, especially one with 8 or so characters to develop like a Trek series. 26 episodes may have forced some of the plot and character development to be too elongated/filled in. 18-20 might be the magic number for Trek. As for number of seasons, 5-7 works for me. If/when we will ever see another Trek, given TV's propensity for all things over-the-top, is another question.