I really don't think that the 3DS version of Super Smash Bros will die out soon at all. In fact, I think that it will live as long as the previous games, alongside the WIi U version of SSB4.
If you don't know, there are still millions of people who still play the classic Smash games like Meleee and Brawl, and the 3DS version also is a really great game. You can take it with you where you like, and you can battle people in tournaments with your own system.
The main reason people think that the 3DS version won't live as long is because they think that the controls are awkward. I really don't find them to be awkward (Note that I own a 3DS XL) in fact, I find them to be really natural and easy to use. Sure, you won't get the comfort you get on a GameCube controller, but it's still really good, for a handheld.