Chris Dixon with a smart post on when to shift focus:
Ask yourself: if you started over today, would you build the same product? If not, consider significant changes to what you are building. [...] You aren’t throwing away what you’ve learned or the good things you’ve built. You are keeping your strong leg grounded and adjusting your weak leg to move in a new direction.
Almost every company I was involved with adjusted the focus of their business model. Keep this in mind.