Giving your customers a soft landing is really important to maintain trust. They loved your product, and proved that the market existed--but it wasn't as big as you thought. When closing down, has anyone explored not completely shuttering the company, but instead continuing to exist as a licensing entity? License the source code itself to your existing customers so that they can continue using it, without all the mes…
One reasonable choice would be to give or sell your former paying customers just a raw code dump, with a notification and a disclaimer of all the included license problems and other embarrassement. Giving it for free would of course be an easy way to avoid unnecessary liabilities.
Who knows, maybe next they want to buy your services to make fixes and new features? Perhaps they might even be willing to sponsor you in making it a proper open source project.
Talk to your customers. They are your most valuable asset.
EDIT: continued the natural train of thought