No, it was not *arbitrarily*adjusted.
Scripture gives at least three different time frames that could be a generation.
There is the 40 years.
There is 70-80 years. (Psalm 90:10)
And there is 120 years (Genesis 6:3)
We just don't know which one it is.
There are also some factors surrounding the establishment of Israel and what that means.
Does it mean the entirety of all the land Israel occupied at it's peak?
Does it include the necessity of Jerusalem being the capital of Israel?
These are factors that we do realize leave the timing up in the air. But it does NOT invalidate the theology of the rapture, the taking of the church out of the world to remove the Holy Spirit's restraining power and deliverance from the Tribulation, the purpose of which is to bring Israel as a nation to accept Jesus as her Messiah, and to judge the unrepentant, unbelieving world.
Since the church, the true body of Christ comprised of believers, is neither of those, then there is no reason for them to have to endure THE Tribulation.
I don't know if you were around at the time as I was, but in the hey-day of the 1970-80's rapture craze, this was the predominant, unchallenged view. It was only after that 40 year window passed after 1988, that the idea of "generation" had to be readjusted. It continues to be so.
Genesis 6:3
6 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,-- this very clearly does not imply a generation, rather the lifespan of a man
2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
3 And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
Psalm 90:10
9 For all our days are passed away in thy wrath: we spend our years as a tale that is told. 10 The days of our years are threescore years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away.Again, this is about the life span of a man - now reduced to 70 to 80 years
your two referrals are not about the length of a generation
In contrast a generation was 40 years - as seen clearly in the 40 years of wandering in the desert for the old generation to die off
Jesus referred to teh 30 AD generation - he says in Matt 23:34-36 You will crucify prophets I send, guilt upon will be on this generation -