A sustainable business model is a profitable business model that will work for the present and the foreseeable future.
Since startups cannot predict the future it might be difficult to know what customers want in the future and validate if the model is actually sustainable.
Reference
Ries, Eric. The Lean Startup: How Today’s Entrepreneurs Use Continuous Innovation to Create Radically Successful Businesses. 1st ed, Crown Business, 2011.
Highlights or timestamps
sustainable business model, one that works today and can work in the future as well
— ^189796 from The Lean Startup