Thats a valid point, however, its worth noting that D&D cosmology is (and has always been) vaguely capped at the Greater God strata of power. With a few possible 'unseen' exceptions...Lady of Pain, Tharizdun, 'The Serpent', AO.
Ever since 2e there has been that tier above Greater God, the Overpowers/Overdeities. They tend to be transcendental beyond normal mortal, or even deific affairs and generally only concern themselves with matters of truly cosmic importance. Very little that even the mightiest mortals can do with the mightiest artifacts can even marginally affect them, and even numerous Greater Gods or entire Pantheons amassed cannot overwhelm them or even seriously challenge them. Only under the most incredible and truly extreme circumstances could a mortal even inconvenience or gain a momentary victory over them. They generally only concern themselves only with what happens on their own plane (or material world), but within that world they have virtually infinite power.
The typical one is an omnipotent creator deity that created a material world and the original deities of that world but now is so hand-off that they barely notice things happening on a mortal level, and only significant strife or discord on a divine level or something that truly imperils the very existence or fundamental balance of the world itself would be endangered, and then they are capable of massive unilateral action such as creating new Greater Gods, stripping gods of their power, creating whole new races, creating worlds, totally rewriting the rules of magic, generally edition-level changes (since this level of deity has been hauled out to explain the 1e to 2e shift in Forgotten Realms, the 2e to 3e transition in Die Vecna Die, and the 2e to Saga to 3e transition in Dragonlance).
The ones I know of: Highgod/Chaos on Krynn, Ao of Realmspace, The Lady of Pain, The Serpent (aka Mok'slyk), the Dark Powers of Ravenloft
The Serpent one is problematic since it is the only being of this scale that really acts beyond one world or plane, it appears to be native or home in Greyhawk but thanks to that notoriously bad module Die Vecna Die it meddles on the Outer Planes and in Ravenloft, and is stated to be of the same origin or class of being as the Lady of Pain herself
I would definitely not say that Tharizdun is of that scale, he was explicitly given Divine Rank 11 (low-end Intermediate Deity) in his 3e writeup in Dragon #294, although from what I gather 4e implies he's a little more powerful than a typical midrange deity.
In other words, I don't care if PC's are 100th level, they generally shouldn't be able to directly confront one of these things in any edition. They are able to make and unmake entire planets and pantheons, annihilate entire pantheons, rewrite the rules of magic, and generally exist to be an in-game form of DM fiat.
At best, they could use very powerful artifacts and/or truly unique circumstances to achieve a momentary advantage such as to escape it's wrath (like staging an escape from Ravenloft), distract a particularly mentally weak one (like imprisoning Chaos in the Greygem), or find a reason to receive a boon so great that it surpasses anything that could be won in battle (like Ao granting Midnight status as a Greater Goddess during the Time of Troubles so she could replace the slain Mystra, that's a mortal spellcaster suddenly becoming the second most powerful deity on the planet thanks to very unique circumstances with an Overdeity)