All right, that's kind of a loaded statement. First thing, no one here is a fortune teller. We're working in numbers and probabilities. That being said...
It seems that the main thrust of your statement is that it's better to make predictions based on the 1) recent performance of players (trends), rather than based on 2) Defense & player position rankings.
My response to that is that any good analysis of how a specific player will perform has to be based on a combination of those. You have to consider the player you're starting, how they've been doing recently, and the matchup that their team is facing.
If you prefer to think of it in terms of "trends", defensive rankings are really nothing more than the trend of a defensive unit in stopping a specific aspect of the offenses they face. If a defense has been very succesful in stuffing the run all year, odds are Julius Jones isn't going to be the guy to break that trend, regardless of how he's performed recently. You really have to take both into account, then make the decision based on that.
Obviously there are a lot of other factors that enter into the old Start/Sit equation, but unless you're talking about a stud (never sit your studs!), you can't afford to ignore defensive rankings for one of your player's opponents, regardless of how well he's been doing recently.