If words mean what they meant in the room, “this generation” is not “this kind of people forever.”
The phrase in the prophets often marks a doomed contemporary people. Jesus loads it with blood-guilt from Abel to Zechariah and says it will land on the people in front of Him.
Matthew 11, 12, 23, 24, and 26 keep the phrase attached to signs, judgment, and seeing the Son of Man. Chapter 24 makes it explicit: this generation will not pass until all these things happen.
A generation in biblical reckoning fits the span from John’s appearance to AD 70. That is not a trick. It is how the book teaches you to read clocks.
The clock Jesus set has run. The New Creation is what remains after that generation’s world ended.