I think it's fair to say that for those failed products/companies, the reason for them failing is a bit more nuanced than "because the core service already exis

I think it's fair to say that for those failed products/companies, the reason for them failing is a bit more nuanced than "because the core service already exists elsewhere". Competition always exists so you're almost never the sole provider of a particular type of service. If you fail, you were likely out-competed, even if it's in a non-core area like marketing or popularity.