Psalm 71:9: Old Age and God's Presence
Psalm 71:9 turns failing strength into a prayer rather than a verdict. The aging speaker asks God to stay, not to restore his youth or guarantee his safety.
John 14:18: Not Left as Orphans
Jesus does not answer the fear of being left alone by promising a feeling. He promises himself. John 14:18 rests the disciples' security on his word that he will come to them.
Psalm 71:9: Loneliness and Failing Strength
Psalm 71:9 gives words to the fear of being abandoned in old age and treats that fear as something to bring straight to God. The speaker asks not to be rejected when strength fails, and the asking rests on who God is rather than on what the speaker can still do.