If David's plan worked we'd be successful, and in football, like any other business, that brings predators. The only way we could hang onto good players would be paying them significantly more money. And the only way we can get significantly more money, without outside investment, is by selling good players.
I understand the idea is to buy cheap, sell dear and build up funds over time, enabling us to retain the core of the squad. But the whole plan surely relies on selling your best player/s on a regular basis, and that can have a huge effect on the rest of the team. Ambitious players will always be looking elsewhere.