By now it is doing well. Every console that sells 15+ million per year is a success
And it is likely to keep this pace until 2019 ends, with pokemon, smash, animal crossing, and other announces.
They are releasing their biggest IPs to keep it selling, but they will become more rare eventually.
So, I dont think that is so obvious that they will still be successful after 2020.
But if they bring price drops and revisions (specialy for higher battery life, or even portability) and ocupy the 3DS market .they can still keep the momentum for much more time and pass the 3DS+wii U market easily.