But certainly there was never a time when I did not exist, nor you, nor these lords of men; nor certainly shall we not exist, all of us, henceforward

Kṛṣṇa affirms the eternity of individual existence. The triple negative na jātu (never at any time) + na āsam (I did not exist) + na (not) covers past, present, and future. The ātman of each is nitya (perpetual).

The list: aham (I/Kṛṣṇa), tvam (you/Arjuna), ime janādhipāḥ (these lords of men) — all are eternal consciousnesses, without hierarchical distinction in their essential nature.

Sarve vayam (all of us) includes even the enemies. The argument is not that they will not die, but that death does not end existence.