At higher levels it takes 3 hours a day to replace water wells and wheat fields. Eventually that will scare away most players. Replacing everything with silos and watermills would cost over the 50k gems for a medium size economy.
Unlike a Veteran general, silos and watermills don't give players any advantage over other players, other than that players with less time can keep playing.
Or maybe make them drop as reward from the adventures everyone skips, like secluded experiments.