The problem with your question is that you link the year '70 C.E.' with this prophecy of the Israelites never being kicked out of their land. The problem is that the Israelites were rejected by God before that date, so any further fulfillment of prophecies on the 'Israelites' would be referring to spiritual Israel - the anointed Christians. The literal Israelites were rejected in 33 C.E., with spiritual Israel taking over in that same year.
The initial fulfillment of Amos 9:15, according to JW publications, was applied to spiritual Israel (w54 149; si 150).
I have also found agreement on this point in non-JW publications:
Ver. 15.—The blessing shall last for ever. They shall no more be pulled up. This was not true of the literal Israel; it must be taken of the spiritual seed, planted in God’s land, the Church of Christ, against which the gates of hell shall not prevail. ‘Lo,’ says Christ, “I am with you alway even unto the end of the world” (Matt. 28:20)
-- The Pulpit Commentary: Amos, H. D. M. Spence-Jones, Ed.
Although the postexilic community experienced a partial fulfillment of such promises as are made in 9:11–15, 157 their ultimate fulfillment awaited New Testament times. Jesus Christ in the line of David was the manifestation of God at work to change the word of wrath to the word of mercy. He founded the church, a God-blessed people, the Israel of God (Gal 6:16). He charged them to make disciples of “all the nations.” This part of the promise (Amos 9:12) is in process of fulfillment. The text, with its first-person verbs referring to God’s actions, makes clear that God is the one who restores, builds, plants, and blesses. It will not be by political coup, social revolution, or military maneuvers that Israel will regain its ascendancy. It will be by the coming of the Lord, who will heal his people and their land.
-- Smith, B. K., & Page, F. S.: Amos, Obadiah, Jonah; The New American Commentary
The blessings that the prophet pictures here are far greater than those experienced by post-exilic Israel. In the New Testament Amos’s picture of the rule of the Davidic king over the nations is applied to the expansion of Christ’s kingdom among the Gentiles (Acts 15:13–18; cf. Eph 3:6). No doubt there will be an greater application in the future (cf. Rom 8:19–23; 1 Cor 15:24–28; Rev 22:1–5).
-- Fleming, D. C. (1994). Concise Bible commentary