The moat
Switching costs (historical books, tax history), brand at tax time, and scale in lobby/compliance around the tax franchise. Network effects limited; data advantages real.
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.