Except that nobody ever does this. You can set up a smartly placed sentry and a dispenser right around the edge of the grain silo on the second point, first stage of dustbowl and dominate an entire team. And by chance it does get destroyed, your dispenser is still there to help you build another. The only real problem are spies, and usually a friendly pyro or engie critwrench takes care of those (or they let themselves be known by taking out the snipers that are usually around in the back).