Ok, i think i can agree with that sentiment to some extent.
I feel that Nioh is a much better game, because it is a complete game, completely tested out, complete content, and in general, it doesn't have game breaking glitches and is a much balanced experience overall (well, as balanced as something with diablo-style loot system can be anyways).
While i think nioh could use some things from dark souls that didn't use (i'm being really specific here, i mean just weapon and armor upgrade and loot system in general).
If we're going to compare, the only Ninja Gaiden game that stands against NiOH in just being a fully fledged out and complete game, is Ninja gaiden Black, and that game is what? 13 years old right now.
But i still enjoy the style of combat and approach of ninja gaiden more, and i think it does things that not any other action game series do, and i'm really glad NiOH is a thing on its own, even though i hate the loot and upgrade systems in the game, i still feel that the game as an action game stands out among the rest really well.
Having said this, i think current Team Ninja can nail the next ninja gaiden and make a perfect game, if first, the project is handled by fumihiko yasuda (director of nioh, and also the responsible from ninja gaiden 3 razor's edge), when i think that NG3RE came out less than a year after Vanilla NG3, i think of the potential that Fumihiko has handling the team by himself from the beginning. It is also worth noting that TN took lessons in enemy design, and also about learning from fan-feedback in this period, which is also something important.
If Ninja Gaiden 4 is a project handled with care, time, and budget, it can be something great, better than any previous ninja gaiden game, the potential is there.