Everything a customer has to pay, redo, or risk to swap your product for a competitor's, in money, time, retraining, and migrations.
When a Fortune 500 swaps SAP SuccessFactors for Workday's HR suite, the project routinely runs 18 to 36 months and costs tens of millions in implementation fees alone, before counting internal hours. The switching cost is the entire reason multi-year enterprise renewals happen quietly: nobody wants to start that project again. Workday's revenue retention sits above 95% as a direct result.