It's not inevitable, not in the slightest.
For starters, where could the possibly take the franchise from here if not a reboot? The amount of content in this game is insane, and honestly insuperable. The man hours grow exponentially with each entry to the series.
Second of all, a new Smash hinges on the NX being successful. WiiU proved that Smash is no longer a franchise that can sell consoles the way it used to. And considering all the news and rumors surrounding the NX, I wouldn't set my hopes too high:
http://www.mcvuk.com/news/read/ubisoft-we-believe-nx-will-recapture-the-wii-audience/0168585
Nintendo has a lot of faith in the NX, but they also had a lot of faith on the WiiU, and look at how that turned out. I can tell you one thing though, if it's meant for casual appeal (which all signs point to yes), you can count me out for sure. Which is the general consensus among most gamers. Nobody wants another gimmicky Nintendo console which they'll give lackluster support to.
More than anything though, the success on the NX will depend on 3rd party support, and until Nintendo manages to turn that boat around, I doubt we'll be seeing anything promising from their next console. Simply look at the sales of Nintendo consoles to get a hint. They cut in half each gen. Lighting struck with the Wii, but I doubt lightning will strike twice. The Wii's retention rate proves that, and VR is the hot new item these days, which Nintendo has vocally declared they're not too keen on trying out.
In short, don't count your chickens till they hatch. A new Smash, like the success of the NX, is NOT "inevitable."