Because if there was only a single source (for example if the other one was out-competed), they'd have to pay 30% of their revenue for the privilege of being in the FabStore.

This is already happening. The leading edge node wafers cost a fortune compared to older nodes. TSMC has limited capacity, as it takes years to bring new fabs online, and with competitors struggling they have great pricing power. Maybe why their revenue has roughly tripled over the last decade.