Greninja was never top tier. He just destroyed For Glory in the same way as Yoshi and Mac.
He sadly got hit harder, probably because his win rate was even better. Hydro Pump was bonkers but nothing else needed to be touched.
Meh, doesn't matter. In most respects he's actually better off now. He misses Hydro Pump, which was pretty much broken, and an overpowered Usmash, but that's it. He now has better pokes (Ftilt, SOB aerials), stronger smashes and a combo Dthrow. I can't really justify prepatch Hydro Pump's existence so I'm happy with how Greninja turned out, aside from Usmash being slightly overnerfed.
I have a lot of vision for the character. I think he's significantly, and I mean significantly, better than most people think. I'd have dropped him if I didn't believe that, and put all my time into Sheik, but she's still my secondary because I'm constantly finding new ways to push Greninja. MUs that I once thought were near-unwinnable have gotten so much better with a little work.
I would bet money on Greninja, Peach or Pac-Man, maybe all 3, having the same journey as Brawl Olimar.