The moat
Switching costs in CUDA, developer mindshare, and full-stack systems (GPU + NVLink + InfiniBand/Ethernet + software). Scale in leading-edge wafers via TSMC. Brand among engineers is cult-grade.
Pain, risk, or retraining required to leave.
The product gets better as more people use it.
Patents, data, licenses, process knowledge, regulation.
Fixed costs spread over a volume rivals cannot match.
Structural ability to be the low-cost producer.
Pricing power from trust, habit, or identity.