There are actually three issues to address here. The first is the number of people in heaven. In Revelation 7 and 14, we are told about a group of 144,000 people that will always be with the Lamb. If we take these descriptions of this group literally they would be only Jewish male virgins. This would mean means that Peter (not a virgin), the Blessed Mother (not a male), and Jehovah’s Witnesses founder Charles Taze Russel (not a Jew) would not be in heaven.
There are other problems with the Witnesses’ thesis that these 144,000 will be the only people in heaven. The book also shows 24 elders that surround the throne of God (Rv 4:4). This brings the grand total to 144,024. This number is increased dramatically by Revelation 7:9, which states there is a countless multitude before the throne. The number 144,000 is likely only a symbolic reference to the New Israel, arrived at by squaring the number of the 12 tribes of Israel and multiplying by 1000.
As to the question of those left on earth, Jesus never speaks of having two flocks, an earthly and heavenly class of followers. Jesus has one bride, and she is not divided into two camps. Those with their name in the book of life enter the New Jerusalem (Rv 21:27) and those whose names are not in the book of life are thrown into the lake of fire (Rv 20:15). This does not leave room for a middle class. All of God’s people are together in the New Jerusalem, not part here and part there.
The references in Psalm 37 to "inheriting the land" are directed towards the promised land of Israel, which later became a type or figure of heaven. We are told by Paul in Philippians 3:20, "our citizenship is in heaven." Those who are occupied with what is earthly will "end in destruction" according to the prior verse.
On the nature of this destruction, the JW’s book Reasoning from the Scriptures states that Christendom teaches "hell is a place . . . where the wicked, after death, are punished (and some believe that this is with torment)." It then states the JW position that "they [the damned] are conscious of nothing, they obviously feel no pain" (p. 169). The Bible indicates otherwise: "The smoke of their torment ascends forever and ever, and day and night they have no rest (Rv 14:11); the rich man, "being in torment," calls out, "I am in anguish in this blazing fire" (Lk 16:23-24); "there men will weep and gnash their teeth" (Mt 8:12).
When discussing these points with a JW, make clear that we were created to see the face of God; as David wrote, "As the deer longs for streams of water, so my soul longs for you, O God. My being thirsts for God, the living God. When can I go and see the face of God?" (Ps 42).
In all charity, the Watchtower’s idea that all but the 144,000 will live in paradise apart from God for all eternity amounts to a promise of an eternal petting zoo instead of the Beatific Vision. You must reawaken in them the desire for the union with God promised to all in Christ Jesus.