The issue isn't power creep.
The issue is cool/logical power creep.
Take Hero X.
Hero X has Air powers.
Hero X encounters a problem. Gets his ass handed to him. Survives. Needs to figure a solution.
Lets say he was defeated with fire powers.
He could:
A) Go learn water powers.
B) He could figure out a creative way to use Air powers to defeat the fire powers.
Method A is "logical" but it is ultimately boring.
Method B might be Hero X learns that Air powers involve moving the air with a form of telekinesis where as Fire powers is the increasing of energy in the individual molecules. Using his ability to manipulate air, he figures out he not only can move air, but can make air go 'still'. In fact, he can make the air go so still that he can basically stop ALL movement. He can reduce air to 0 Kelvin. In effect, if air is in contact with something, he can 'freeze' the object. However, he still is limited to air, so he can only affect the surface that is in contact with air.
He can't affect things in a vacuum. He can't affect the inside of solid objects. He can't affect something underwater.
We have 'Power creep', but the creep is a logical extension of the power the hero already has, as well as includes limitations that the villain can then exploit.
The beginning after the end. It's a manga and... I just... love and hate it.
I'm okay with the story, but the problem is, the hero is the ultra-cool I got every element and I got the super powerful core and I have ultra power and... Okay. Sure. You did a good job developing the magic system, and all this crap.
Why the fuck hasn't the Hero just murdered everyone?
If I had his suite of powers and his knowledge from the previous world he was reincarnated from, day one, I'm making heavy water, day two, electrolysis, day three, NUKES.
I have Water, Air, Fire, Earth, Lightning, Animal Spirit Core, Blah Blah Blah...
Ultimately, I'm sitting there wondering, "Why haven't you just won? You had ANTI-GRAVITY in your previous world, for fuck's sake! " But We're not supposed to think about that.
The problem is, guy has to be the ultra cool guy with his large number of powers, but that's not heroic. But if he has one power that he can push to new limits, or he's sort of average, but he just has cool knowledge, that's more interesting. We want to know HOW the hero wins. We KNOW he is going to win. The more power you give the guy, the more boring it gets, because we come to that question: Why don't you just blow the bad guy up?
Of course you can just give the villains more power, but then you get into an arms race where increasingly odd nonsense takes place and every villain has to be stronger than the last.
The Average Guy winning is far more inspiring than Special Snowflake wins, however, Special Snowflake is fine if Average Guy EARNS it. We want the Hero to become special snowflake and just fix everything, just understand once he does, the story is over.