Hard do say something. If you really want to shutdown the project, send a notification email if you have email of the users or show a notification on the website about the shutdown, so they can save their data etc.
Or you can handover the project to someone else, or it's unlikely probably but make it open source and find maintainers.
These are a few options came to my mind. It's more important how many active users you have then number of users. However, 17K is a good potential, you should decide very carefully.
Good luck.