I feel like a huge mass of people will hop on Rain when he is released, but I get the feeling that most are saying they will main him just because he is "that ninja that got left out." Of course there are die hard Rain fans out there (like myself), but for a lot of people I believe that if Rain was included in the original version of the game, he would be a lot less popular across the internet.
People always want what they can't have, which in this case was Rain until he was just announced. Expect the Rain Dancers to slowly dwindle down in the weeks following his release.