Random thoughts -
1. After you get to the playoffs, winning the WS involves a lot of luck.
2. I think there's a limit to run prevention - the resources that you have to commit in order to prevent a run starts to rise as you prevent fewer runs. Eventually, you start to reach a place where more of the runs become unpreventable. So, practically speaking, defense is still important, but you shouldn't use it as the only way to construct teams (see 2010 Mariners). I don't think it's an issue of scarcity so much as it is an issue of diminishing returns. Similarly, there's probably an upper limit to the runs that you score.
3. Iglesias is nowhere near Lowrie yet (and may not be in the future) - his only offensive value comes from his ability to make contact, he doesn't have good plate discipline or power. There probably will be a time when Iglesias is more cost-efficient (and those more valuable in a per-dollar sense), but I feel like Iglesias won't ever reach Lowrie's level in terms of total value.