As I understand it… “the world” that was “passing away” was the old covenant world. In Christ the new world/age was taking hold, or as the writer of Hebrews has it… In that He says, “A new covenant,” He has made the first obsolete. Now what is becoming obsolete and growing old is ready to vanish away. Heb 8:13
Those who survived the conflagrations of the Roman-Jewish wars of Ad66-70 [of which btw such insurrections were not restricted to Palestine alone] were those who survived “into the age about to come”. The coming age was/is not some esoteric or ethereal “heaven” after death, but rather… the fullness of the new covenant age begun in Christ.
I like for example N.T. Wright’s rendition of Mk 10:29-30…
There is no-one who has left house, or brothers, or sisters, or mother, or father, or children, or fields, for my sake and the gospel’s, who will not receive a hundred times over, now in the present age: houses, and brothers, and sisters, and mothers, and children, and fields — with persecutions; and, in the coming age, the life of the new world.
N.T. Wright: Jesus and the Victory of God p. 402
The “present age” was the then Mosaic era of law-righteousness, while “the coming age” wherein dwelt “the life of the new world” was the new covenant age — the age from Christ’s cross forward. The real inter-testamental age was the 40yr overlapping period Ad30-70; an age emulating Israel’s exodus out of bondage, where those of faith came into the fullness of Israel’s redemption.
IOW, they grasped “the life of the new world” IN THIS LIFE which was none other than the “eternal life” as Jesus defines it in Jn 17:3, that is, as a PRESENT reality, or as he puts it in Jn 10:10… “life to the full” aka PEACE as per Jn 14:27.