Melee I think will always reign as the most popular in the series. Sm4sh has done really well but I think that it will have a similar longevity to brawl, in that it is really popular when it first comes out but proves to have little staying power. Melee is just such a perfect example of a beautiful accident, the meta is just so deep and there is no endgame in sight. Don't get me wrong I like sm4sh, a lot, I just don't know how I will feel about it two years from now.
I feel the big advantage Sm4sh has over something like Brawl is that there is a lot of potential for DLC keeping the game fresh for a long time. I do agree in that it will likely fall to the wayside eventually, but it will be mitigated at least somewhat by consistent updates to content.