It must kind of suck to be a US show. They don't get time to shine really. In the UK, things continue until it gets bad ratings, but in the US, if its underperforming by a tiny bit, they pull it, and replace it with something else... which gets cancelled anyway and on and on and on. It's more beneficial to let a season air out surely, because they keep replacing cancelled shows with worse performers.
Lets look at history itself, shows such as The Big Bang Theory and even Friends first season and the first 2 seasons of Seinfield, they only did "OK" for their channels at the time. They were given a shot and all became huge. Now, imagine how different the world of TV would be if Friends and Seinfield was cancelled after 1 season.
CW actually has the best approach to their shows, as far as I'm aware, they mostly air out their shows to finish them and then pull them.
MLS is the first cancelled right? It will be replaced by something that probably does as badly, it's just an ongoing cycle!
What I found interesting is a post on TVBTN (zap 2 it in other words) that around 80% of freshman shows end up cancelled, whereas in the UK, they give it time. Here are the freshman shows last year which are back this year...
CBS - The Millers, Mom.
ABC - SHIELD, Resurrection, The Goldbergs
NBC - The Blacklist, Chicago PD, About A Boy
FOX - Brooklyn Nine-Nine, Sleepy Hollow.
CW- Reign, The Originals, The 100.
Lets not forget The Millers is owned by CBS, it wasn't doing too well, so that maybe would have been cancelled anyway. So Mom would be seen as quite special to them... hence giving Mom the post BBT slot for a while? And The Goldbergs was lucky to get renewed, luckily now for them, it has done much better in its new slot.
And here are the ones cancelled...
CBS - Intelligence, Hostages, We Are Men, Friends With Better Lives, The Crazy Ones, Bad Teacher.
ABC - Lucky 7, Trophy Wife, Super Fun Night, Once Upon A Time In Wonderland, Betrayal, Back In The Game, Killer Women, Mind Games, The Assets, Mixology, Black Box (this one's tricky, it premiered in-season but aired through the summer).
NBC - Ironside, Welcome To The Family, Sean Saves The World, The Michael J. Fox Show, Dracula, Believe, Crisis, Growing Up Fisher.
FOX- Rake, Dads (I can't find the rest of the cancelled shows)
CW - The Tomorrow People, Star-Crossed.
Pretty harsh odds. Overall CW has the "best" year in terms of keeping shows. It lost 2 new ones, it renewed three new ones and renewed older shows... Beauty and The Beast, Hart Of Dixie, Arrow, Supernatural and The Vampire Diaries. And this year so far, it looks like Arrow, TVD, The Originals, Supernatural (maybe a final season), The Flash, Jane The Virgin? may get other seasons... so I'd say they are doing pretty well.