Albert Barnes Commentary Jeremiah 31:37

"Thus saith Jehovah: If heaven above can be measured, and the foundations of the earth searched out beneath, then will I also cast off all the seed of Israel for all that they have done, saith Jehovah." — Jeremiah 31:37 (ASV)

All the seed - Unworthy members of Israel may be cast away, but the people shall never entirely cease to exist.