In the same way that someone can also start a new city at any given time. Yet, the price of real estate in Manhattan remains expensive. Blockspace on major pre-existing chains is valuable because 1) the ability to compose with other applications running on that chain, and 2) higher value of native assets increases the security of consensus in PoS networks.

Again, this isn't just hypothetical. Today, users pay an orders of magnitude premium for Ethereum block space compared to other chains. That's because Ethereum has much higher value applications to compose against and better security guarantees around consensus.