Yeah, based on what he's said
during development this time, I think he's serious about retiring this time. That being said, the series definitely won't be over, as long as Nintendo likes money and good press.
I'm mixed though. I doubt the Smash Formula will suffer. Drastic gameplay changes are
not something I'd expect. If it's not broke, don't fix it, you know? But in some ways, it's a blessing and a curse. Character choices come to mind. I'd worry for characters no longer in the public eye. Sure, Sakurai doesn't give a hoot about that, but would
other developers? Would not only the Ice Climbers stay cut, but characters like Jigglypuff, Greninja (at the time of Smash 5), the retros, and maybe Ike (though to be fair he's a bit safer in my opinion in this sense) be lost in the process?
On the other end as far as characters, at least the likes of Chrom and Ridley would have a chance later on.
As far as a more competitive aspect, I'm uncertain. Sure, Nintendo seems to love the competitive scene now. All those tournaments and such. But at the same time I'm worried that they may make the series suffer a Mario Party effect. Only time would tell for this.
But then again, I'd just hope they'd not Pink Gold Peach us.
tl;dr: Only time will tell here. It can go either way, in my opinion, on various aspects of the game.