Yes. You're right. Your past performance isn't indicative of future success.
I have no idea if Adrian Peterson will be a better RB this year than Knowshon Moreno. Who knows? This is a new year.
Individuals do make up teams. And if you can tell how good an individual is by past performance, which everyone does, then you can look at the group of individuals, or team, and project the future by their past. I knew the 90's Cowboys were going to win another Super Bowl after 92 by their collective performances in the past. It wasn't a level playing field in 93, even if the fans of the also rans wanted it to be.
Ya know when you really can't tell by past performance? When it's August and your football team hasn't been good in a really long time.