The 9th season was more of an added bonus. The writers and the cast weren't sure whether CBS would renew them for another season, so the first half of season 8 was written as if it were a final season. When they got the renewal, they could (hypothetically) work out all of the details in which Ted met the Mother. Instead, they showed the separate stories of how Ted and the Mother met. This is a very "classic HIMYM" approach to the final season, where they show the back stories of the Mother before Ted meets her. Had the show ended in season 8, we wouldn't have had any sort of closure to the show; Ted never would have met his future wife. But what the writers could have done better was put more focus on the mother and less on the others. For example, there is an entire episode where Marshall spends his time on a bus telling Marvin stories in rhyme. This episode had absolutely no purpose. Instead, they should have had a flash-forward episode, where they show the Mother and Ted with the rest of the gang
The first half of season 8 was some of the best writing in the entirety of the show- from both the comedic and dramatic side. The second half of season 8 and season 9 thus far have not been nearly as impressive as any other seasons/episodes. Though the structure would've had to been changed to end the show in the Spring, the Final Page episodes were the perfect ending to the show. Marshall and Lily's story culminates with Marvin and the struggle between responsibility for him and maintaining the relationship with their friends. This is shown in the first half of season 8 with the godparent episode and would've wrapped up nicely with the two of them struggling to enjoy time without Marvin while supporting Ted at the party for his building. They learn that they can manage both parts of their lives. There's not much that needs to be said about the Robin/Barney side of things. The limo scene with Ted and Robin was so emotionally loaded it was perfect for a show's finale. Ted's lines about making an ass of himself and how it led to their friendship was the perfect way to finally close the case on the Ted/Robin possibility. And of course, "The Robin" was pure magic and a shock but left everybody satisfied. Finally, when it seems that everybody's lives are dapper except for Ted, he meets The Mother at the celebration of his building. It's symbolic genius, all his hard work of searching for the one, getting over Robin, paralleled with the completion of his first building- he finally reaches the top and gets what has been waiting for him all along. The show isn't called "WHO Your Mother Is" for a very good reason: it's all about the HOW. All the good times, bad times, memories, laughs, and tears that made the show and how they lead to the point where Ted finally meets the one.
I always wanted the show to end with a season showing Ted meeting the mother at the beginning of the season and then dating her throughout the final season. Having a whole season of 24 episodes take place over one weekend sounds like it may mess with what the show is.
Also, it should have been renewed due to having very high ratings, still. Some people are projecting that this could be the second biggest final season since Friends (accounting for overall TV ratings decline).