I'm starting to think Gren's sadly not coping with the power creep in this game.
There hasn't been a power creep in Ultimate though. On the contrary, most of the top tiers from Smash 4 were nerfed and even many non-top tier characters got some of their best moves nerfed.
So that leaves the general new mechanics which affect the whole cast:
- weaker shields and airdodges
- stronger edgeguarding
- lower aerial landing lag
- dash cancel into tilts
These changes mostly benefit characters with great mobility who do not need to rely on shields that much, with strong spacing aerials which will become safe on shield thanks to the landing lag reduction, with great tilts and strong recoveries. And Greninja is among the best in all of those categories. So in theory Greninja should do better in Ultimate than it did in Smash 4.
But even if Greninja ends up being weaker, that might be a blessing in disguise. Sakurai will be looking out for the strongest characters from the start and hitting them hard with nerfs. And due to Sakurai's lack of understanding of the competitive scene, it is not uncommon for him to overdo it with the nerfs. It already happened to Greninja in Smash 4 and not just once. So the best thing for a character is to have powerful tools that are hard to learn initially, so that it goes under the radar at first and avoids the initial waves of nerfs. And this is pretty likely to happen with Greninja.
Greninja is in that perfect spot of being a high tier character that does not attract too much attention with overpowered moves, but has really fun gameplay and deceptively strong options, many of which surpass what most top tiers have. It is just held back by its lack of aerials with fast startup and long lasting hitboxes. Also its disadvantage state is not the best. But pretty much everything else from mobility to combos to kill setups is top tier level.
And unlike other fast characters it is often compared to like Sheik, Fox or ZSS, Greninja is the only one with a good kill throw with a long grab range and buffed frame data, great disjointed aerials for spacing and a projectile that can either deal chip damage or kill, is transcendent when fully charged and cannot be absorbed. So I do think Greninja has enough tools to hold a viable spot in the Ultimate meta and separate itself from the rest.