The reason it is a new song is two aspects:

1) the Lord newly died so men are redeemed,

"And they sung a new song, saying, Thou art worthy to take the book, and to open the seals thereof: for thou wast slain, and hast redeemed us to God by thy blood out of every kindred, and tongue, and people, and nation" (Rev. 5.9).

and 2) 144,000 are brought home first,

"And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth" (Rev. 14.3).

Overcomers are not all believers just those who were watchful, prayerful, and kept the word of His patience and can include martyrs. All men overcometh just not at the same time.

My Life Application Study Bible says 144 cubits equals 65 meters so one cubit equals 17.7 inches not 20 inches.

Nothing happened in the year 2000.