Adam Clarke on Genesis 17:8

Everlasting possession - Here עולם olam appears to be used in its accommodated meaning, and signifies the completion of the Divine counsel in reference to a particular period or dispensation. And it is literally true that the Israelites possessed the land of Canaan till the Mosaic dispensation was terminated in the complete introduction of that of the Gospel. But as the spiritual and temporal covenants are both blended together, and the former was pointed out and typified by the latter, hence the word even here may be taken in its own proper meaning, that of ever-during, or eternal; because the spiritual blessings pointed out by the temporal covenant shall have no end. And hence it is immediately added, I will be their God, not for a time, certainly, but for ever and ever. See the note on Gen 21:33.