Fans universally agree that Ted/Alice would be a horrible pairing, fully supporting the official pairing.
Ted/Alice was originally the Official Couple of the series but due to Executive Meddling, namely the producers thinking that Bob/Alice would be more popular, that route was chosen. The writers still wanted Ted to be with Alice, so they tried to make the Ship Tease as subtle as they can to avoid it being cut.
Ted/Alice was originally a popular fanon pairing, but then it got a Relationship Upgrade and became an Official Couple (though it works for the best if Bob is no longer in the picture at all).
Ted and/or Alice become The Scrappy later on in the story and therefore the pairing loses its popularity.
Ted/Alice may have been the original intention, but the writers liked Bob/Alice a bit more.
Ted/Alice did become an Official Couple but then they broke up later on because the writers did not want to do anything with that pairing anymore. However, fans still liked that pairing and the reason(s) why Ted and Alice broke up was very poorly received.
Zig Zagged: Bob/Alice is the Official Couple, but is broken up in favor of Ted/Alice. However, the way Ted/Alice gets a Relationship Upgrade is clumsily done, causing the pairing to lose popularity and many fans to jump ship to Bob/Alice. Then Ted/Alice gets broken up, too, leaving Alice single and with so many Ship Tease moments afterwards with both Bob and Ted that the popularity of Bob/Alice and Ted/Alice in the fanbase varies wildly from week to week.
Averted: The fans are perfectly fine with Bob/Alice.
Discussed: "You know, Alice, a lot of people feel that Ted is a better match for you than Bob".
Conversed: "Yeah, I know that Bob/Alice is the canon pairing of the series, but I've always been more partial to Ted/Alice."
Ted's fanbase is a case of Draco in Leather Pants who would actually be a terrible choice for Alice. Basically, if you think Bob is bad for Alice, then Ted is a lot worse.
The writer gives the fan-base what they want, by having Alice cheat on Bob with Ted.
Reconstructed: The writer realizes that some fans see Alice's relationship with Ted as much more organic than her relationship with Bob and that the fanbase is merely taking that relationship to a logical conclusion. As a result, the writer allows the fans to make up their own minds who Alice ended up with after the finale.