Except that the superiority, at this point, has
nothing to do with the product itself. It is primarily—possibly solely—due to
who else uses it.
That is not something that WhatsApp chose, or designed, except in that they hoped that everyone would be using it.
This is not a free market, where the product in question is something that one simply purchases and continues to purchase, and can switch to any other product freely. The products in this space are, to some extent, non-interchangeable (Facebook, Twitter, and WhatsApp may all offer online communication with other users, but if the users you want to contact are only on one of them, that is the only one that is useful to you), and there is a significant barrier to exit.
So a product that no one would have willingly chosen to start using may still end up being used long after it is, objectively, inferior to every other offering in that product category—so long as the people using it still need to communicate with each other, and cannot easily coordinate a mass exodus to a different product.