Why not focus on existing cities rather than building new ones?

After more than 15 years of stagnation, per capita GDP growth is paramount. Data from the Resolution Foundation shows that our last new city, Milton Keynes is far more productive than anywhere else in the UK except London. Conversely, adding homes to existing lower productivity places might be far less cost-effective because: 1. The infrastructure to make those places more productive (mass transit, better road networks, phone masts, sewerage, cheaper energy) is rarely guaranteed to be built alongside housing. 2. When it is built, it's painfully slow and expensive because people already live there and have huge veto powers. 3. You can't capture the land uplift value to pay people off. Given the crisis we're in, pitting efficient, at-scale greenfield build-out that pays for itself against expensive retrofitting via town extensions makes little sense.