Please look up the word Hades and sheol before making the assumption that hell is referring to the grave, because it is not.
qeber (hebrew) and mnemeion (Greek) are the two words interpreted tomb, grave and sepulcher. Sheol/Hades are defined as the place of departed spirits, the underworld:
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Strong's Concordance
hadés: Hades, the abode of departed spirits
Original Word: ᾍδης, ου, ὁ
Part of Speech: Noun, Masculine
Transliteration: hadés
Phonetic Spelling: (hah'-dace)
Definition: Hades, the abode of departed spirits
Usage: Hades, the unseen world.
HELPS Word-studies
86 hádēs (from
1 /A "not" and
idein/eidō, "see") – properly, the "unseen place," referring to the (invisible) realm in which all the dead reside, i.e. the present dwelling place of all the departed (deceased);
Hades.
Sheol/Hades should never be translated as the grave, because this is the place where
the spirits of the unrighteous go at the time of death.
Furthermore, Hades which will be thrown into the lake of fire, is personified, i.e. an actual entity representing the same place.
And while you are correct that Sheol/Hades will not exist anymore, the lake of fire will last forever and so will those who are thrown into it.