I'm not sure "prove" vs. not prove, but if you think that it's possible to simply assemble a winner via FA, you are wrong. There are good "sure-thing" FA signings out there. They do exist. Those guys get top dollar for a reason. There are rarely enough to fill an entire team, or even plug all the holes that a team may have. There's also the problem that there are several teams with the pockets to get such guys (Angels, Dodgers, Yankees, RedSox, and Cubs). If these teams want a guy, they will get him and when they bid against each other, the price becomes exorbitant. As others have said, going crazy in FA to get that one piece makes a ton of sense. Doing so to turn a 90 loss team into a 90 win team is a recipie for having an at best 80 win team with a bunch of really expensive contracts.