Not only killed viewership for the spin-off, but also for reruns of this show.
I don't see this show will really go down in history as one of the really great sitcoms. It had the potential to before the finale, but they ruined it with the way it ended. It will always be a black mark over the show and will affect the way everyone views the entire series from now on, as they'll have this intepretation of events. Much of it will seem completely pointless now, particularly the 9th season and the critically acclaimed 200th episode featuring Cristin Milioti in the main role.
And to think how well received and critically acclaimed that episode "How your Mother met me" was, and now we have this. I really thought it would go out on a high. But really, are there honestly any fans of the series who actually followed it through to the end (and didn't simply skip out on most of the series to only watch the finale) who truly loved and enjoyed this last episode?
People might say "well, I understand why this was done" etc, but can anyone honestly say they thought this was brilliant on the level of the 200th episode and the perfect ending to the series that would mark it as one of the greats?
At least Friends ultimately remains a more watchable series because it didn't pull a stunt like this, even though, for the most part, I enjoyed HIMYM more.