I think people tend to overlook that, yes, the newcomer pool might not be as "famous" as past ones, but that's because the all-stars are ALREADY IN THE GAME. The cover will still be Mario punching Pikachu in the face while Samus tries to dodge Link's sword, and children everywhere will go wild for it.
I'm sure there will be a high profile Third-Party or two to pin some of the marketing on, but just because we're looking at Ridley/Rool/Dee/Retro's/etc for First-Party doesn't negate all of the legends already in the roster. We're certainly not at the "get Steve from Minecraft or the kids won't buy it!" stage of Smash history. Mario, Link, Samus, tons of Pokemon, Animal Crossing, now Inklings... the list goes on. It's still a phenomenal roster that will delight people of all ages without scrambling for non-Nintendo characters.
Also, new content like stages and game modes will be big selling points too. I'm sure some new stuff that takes advantage of the Switch's pick-up-and-play angle will be heavily promoted.