Only its benefits will be everlasting. (for the earthly 2nd class sheep)
w80 7/1 p. 31 - Questions From Readers - Why does the November 15, 1979, issue of The Watchtower (pages 26, 27) speak of the “new covenant” being near the end of its operation, when Hebrews 13:20 speaks of this covenant as being “an everlasting covenant”?
"The Watchtower stated: "Evidently the new covenant is nearing the end of its operation for the purpose of producing 144,000 spiritual Israelites....When the last of these approved spiritual Israelites cease to be ‘men’ because of earthly death and a resurrection to a share in the heavenly kingdom, then the mediatorship of Jesus Christ will cease also. Note that The Watchtower did not say that the benefits of this new covenant would cease. Those who live on earth will benefit in an everlasting way from the fact that this new covenant fulfilled its purpose in producing 144,000 spiritual sons of God who will reign as joint heirs with Christ in the heavens."