Are you saying that those words applied only to those who were alive at the time (of Jeremiah's generation)?
I think I would both agree and disagree.

I believe that it not only applied to them (as the kingdom was "at hand") but to all of us as "
a chosen generation".
Luke 21:29-36 And he spake to them
a parable; Behold the fig tree, and all the trees; When they now shoot forth, ye see and know of your own selves that summer is now nigh at hand. So likewise ye, when ye see these things come to pass, know ye that
the kingdom of God is nigh at hand. Verily I say unto you,
This generation shall not pass away, till all be fulfilled. Heaven and earth shall pass away: but my words shall not pass away. And take heed to yourselves, lest at any time your hearts be overcharged with surfeiting, and drunkenness, and cares of this life, and so
THAT DAY come upon you unawares. For as
a snare shall it come on all them that dwell on the face of the whole earth. Watch ye therefore, and pray always, that ye may be accounted worthy to escape all these things that shall come to pass, and
to stand before the Son of man.
The DAY of the Lord comes as a thief IN THE NIGHT, right? And we know that those who "sleep" sleep "in the night", right? It is upon those that "sleep" that "sudden destruction" (judgment) comes, right?
The kingdom of God comes "not with observation" because the kingdom of God is "within".
We must pass from DEATH (darkness, sleep, the NIGHT) unto LIFE (light, awake, the DAY).
Eph 5:14 Wherefore he saith,
Awake thou that sleepest, and arise from the dead, and Christ shall give thee light.
So I see these words of Christ as having a spiritual application to all who "sleep" (the dead) and "if one died
for all then were
all dead".
Certainly these things happened (spiritually) for those who were alive at the time of Jeremiah, but they continue to happen even today (and into the future) unto all of "this generation" (as a chosen generation) wouldn't you say?