Apparently, people are mad about a certain show's finale.I have only watched a few episodes so I'm not very invested.
But I do find the rage interesting.
Writing for a show is not like writing a book. Lots of people affect the writing, and often, the writers have to scramble to get a script to fit around events that are fixed by other people (like a producer, or the pregnancy of an actress, etc).
I don't know about the creative pipeline for How I Met Your Mother, but I do wonder if the ending was chosen by the writing team, or someone else. It's a tough gig, writing for a tv show.
I've only co-written one story with a friend, and it only worked because we were goofing off and on the same page. Working with a whole team of creatives would probably be too much for me, unless I had total control over the plot.