Which MLB players' October breakthroughs changed their 2021 outlooks?

Projecting players is always a difficult, error-filled practice, but it's even more difficult after a season of only 60 games. It would be a mistake to consider a projection for a player as something written in stone like your blood type or your birth date. Projections are like an endless puzzle in which you never really find out the "right" answer and every tiny bit of data shifts the direction you expect from a player. For players in a season with a relatively tiny number of games, postseason performance has more relevance than usual in setting expectations.

That postseason stats don't "count" in a player's record is an odd quirk stemming from the lack of organized postseasons during professional baseball's formative seasons. The proto-World Series matchups of the 1880s were informal exhibition matches that had to be negotiated between teams, and even the first modern World Series of 1903 was simply a voluntary event agreed to by the owners of the Pirates and the (Boston) Americans.

With the World Series being the official championship of the American League and National League (Major League Baseball wasn't really a single legal entity until just 20 years ago), these games are considered the most important by players and fans. So, we have the strange circumstance of Hank Aaron's 755 career home runs not including the three homers he hit in the 1957 World Series that helped the Braves defeat the Yankees. Why wouldn't performance in the most crucial games against the toughest opponent be taken into account? It's like a world champion skier not putting their Olympic gold medal on their résumé.