I've had so many endless discussions about what is viable, I've come to regard that word as deeply unhelpful to the development process.
My benchmark of MVP readiness is: can we expect useful feedback from a customer by demoing this? Some people want to demo a picture and some want to demo a near finished product.
For commercial products, I think the most useful interpretation is that a "viable product" is one that someone is willing to buy. That means implicitly that it has to provide value for the end user. One could also stretch the goal a little by saying that it means you can build a cashflow-positive business around the product. That implies both willingness to pay/end user value provided and an adressable market that can support operating the business. This latter definition avoids the case where a product fits just one single user and noone else.
My benchmark of MVP readiness is: can we expect useful feedback from a customer by demoing this? Some people want to demo a picture and some want to demo a near finished product.