"This generation" I believe is in relation to the "
last generation" that sees
all the signs that are fulfilled in the last days of Matthew 24 it is not talking about the generation then present although some scholars try to apply this present generation to the destruction of Jerusalem. The proof of this is in the question the disciples asked JESUS in v3
And as he sat on the mount of Olives, the disciples came to him privately, saying, Tell us, when shall these things be? and what shall be the sign of your coming, and of the end of the world? V4-33 list all the signs that must be fulfilled before the second coming can take place. "This generation" is in reference to the
final generation that will be alive after all the signs have been fulfilled! This generation after the fulfillment of all signs listed by Jesus shall not pass away before the second coming is come.
There is no scripture that says that the apostles thought JESUS was coming in their lifetime. Your reading into the scripture things that the scriptures do not say or teach. I am sure they all hoped that JESUS was coming in their lifetime though. There is a difference.
One of the prophecies of the last days is this...
2 PETER 3:3-16
[3], Knowing this first, that there shall come in the last days scoffers, walking after their own lusts,
[4], And saying, Where is the promise of his coming? for since the fathers fell asleep, all things continue as they were from the beginning of the creation.
[5], For this they willingly are ignorant of, that by the word of God the heavens were of old, and the earth standing out of the water and in the water:
[6], Whereby the world that then was, being overflowed with water, perished:
[7], But the heavens and the earth, which are now, by the same word are kept in store, reserved to fire against the day of judgment and perdition of ungodly men.
[8], But, beloved, be not ignorant of this one thing, that one day is with the Lord as a thousand years, and a thousand years as one day.
[9], The Lord is not slack concerning his promise, as some men count slackness; but is long-suffering to us-ward, not willing that any should perish, but that all should come to repentance.
[10], But the day of the Lord will come as a thief in the night; in the which the heavens shall pass away with a great noise, and the elements shall melt with fervent heat, the earth also and the works that are therein shall be burned up.
[11], Seeing then that all these things shall be dissolved, what manner of persons ought you to be in all holy conversation and godliness,
[12], Looking for and hastening to the coming of the day of God, wherein the heavens being on fire shall be dissolved, and the elements shall melt with fervent heat?
[13], Nevertheless we, according to his promise, look for new heavens and a new earth, wherein dwells righteousness.
[14], Why, beloved, seeing that you look for such things, be diligent that you may be found of him in peace, without spot, and blameless.
[15], And account that the long-suffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given to him has written to you;
[16], As also in all his letters, speaking in them of these things; in which are some things hard to be understood, which they that are unlearned and unstable wrest, as they do also the other scriptures, to their own destruction.
What do you think those scriptures mean?