I'm not knocking Bill James and his ideas or trying to say he isn't a great baseball mind. I'm just pointing out that his sabermetrics and defensive spectrum is based only on stats and formulas. I've already shown how those types of analysis often have flaws. Unless you are actually doing something you can't know how hard it is.
You keep bringing up guys like Cabrera and Fielder who aren't very good at the position. Just because they can play it doesn't mean they are good at it. The reason all first basemen are bigger guys is because that's what the position requires. Other infield positions require more speed while first requires a big target. There's a reason about 90% of first basemen are at least 6'2 because guys that are small at the position will struggle more. A prime example is the 5'11 Prince Fielder who is probably the worst defensive first basemen in the game. I wouldn't put Adrian Gonzalez at second but I also wouldn't put the 5'9 Dustin Pedroia at first. The bigger the target the easier it is to make an accurate throw and the easier it is to get an out. I guarantee guys like Pedroia find second easier while guys like Adrian Gonzalez find first easier. It all depends on the type of player.
First basemen are involved in almost every single play where a ball is hit in the infield. They don't just have to scoop throws in the dirt, they have to expect bad throws and be able to come up with the catch whether it is high, low, or outside. Just like at second, there aren't as many balls hit to that side of the infield and first basemen field the least amount but they still have to make the occasional play on the ball hit to them. They also have to be alert in bunting situations and be able to charge and make a play on the ball. Having a good defensive first basemen makes the entire infield better and can save a team in a lot of situations.
You keep bringing up guys like Cabrera and Fielder who aren't very good at the position. Just because they can play it doesn't mean they are good at it. The reason all first basemen are bigger guys is because that's what the position requires. Other infield positions require more speed while first requires a big target. There's a reason about 90% of first basemen are at least 6'2 because guys that are small at the position will struggle more. A prime example is the 5'11 Prince Fielder who is probably the worst defensive first basemen in the game. I wouldn't put Adrian Gonzalez at second but I also wouldn't put the 5'9 Dustin Pedroia at first. The bigger the target the easier it is to make an accurate throw and the easier it is to get an out. I guarantee guys like Pedroia find second easier while guys like Adrian Gonzalez find first easier. It all depends on the type of player.
First basemen are involved in almost every single play where a ball is hit in the infield. They don't just have to scoop throws in the dirt, they have to expect bad throws and be able to come up with the catch whether it is high, low, or outside. Just like at second, there aren't as many balls hit to that side of the infield and first basemen field the least amount but they still have to make the occasional play on the ball hit to them. They also have to be alert in bunting situations and be able to charge and make a play on the ball. Having a good defensive first basemen makes the entire infield better and can save a team in a lot of situations.