We don't sell our apps to other companies but you may be interested to know that our company which operates the largest automated (driverless) metro transit system in the world is highly reliant on FoxPro and always has been since its inception in 1986.
We have developed, (in addition to typical apps such as HR, Job Costing, etc.), highly specialized apps that include:
Maintenance Management System
Distributed Automatic Train Location Display and Monitoring
Operations Incident Logging and Analysis
Service Analysis (including graphic output)
Automated Voice Announcement System
Server Based Integrated Paging System
Server Based Trigger and Messaging System
Train Orientation System
Real Time Data Loggers for Operational systems
etc
All of the above rely almost entirely on FoxPro (from DOS2.6 to VFP6).
We are currently developing an OMS Gateway interface that will communicate directly with a proprietary Object Oriented Database to allow for continued operation of the transit system in the event of a partial failure of the main operational computer.
I believe that FoxPro is robust as any other development tool.