Baseball tried 9 games. Too long to hold interest.
Most sports playoffs determined that 7 is the ideal number for maintaining interest (maximizing revenue) and ensuring the best team advances.
Basketball made the switch and there are fewer random factors there that would cause an upset in the first place.
7 is also good because it guarantees that each team in a series gets at least 2 home games.
I think that the biggest problem with baseball - short of shortening the season by a week or two - is weather. 3 of the 4 DS teams are already in climates that could see 50s and rain (and maybe snow) in the next couple of weeks. (Either way, the weather in Houston/Tampa will be warm).
That being said, I do like the game 163 tiebreaker, and wish that the NHL would follow suit, having a game 83 for end-of-season tiebreakers instead of ROW (or just RW, for this year).
Another playoff Idea that I like (a couple of European Hockey Leagues have this) is a playoff draft - where the 1 seed picks it's playoff opponent.