Why does it say "Yemei Shenoseinu Vahem Shiv'im Shanah"?
Maseches Semachos (3:8): One who dies at the age of 70, this is death of Chibah (dearness).
Mo'ed Katan 28a: The age of 70 is Seivah 1 .
Yerushalmi (Bikurim 2:1): Death at 50 years is via Kares. This is 20 less than a normal life of 70 years; Beis Din above does not punish and give Kares before 20. (If one died at 50, he lost the years for which Beis Din above does not punish - R. Shlomo Siriliyo there.)
Rashi: These years of ours, due to our sins, are [only] 70.
Radak: Even though Moshe lived 120 years, he discusses most people. Lives shortened. People in Galus will not see the salvation, if it is not soon.
Malbim: If our years will be standard, they will be 70.
The Mitzvah "Mipnei Seivah Takum" (Vayikra 19:32) applies to one who is at least 70 (YD 244:1. What is the source from our verse? Perhaps "Seivah" implies one who is older than an average lifespan. - PF) Rashbash (Magen Avos) and Bartenura (on Avos 5:21) learn from David "va'Yamos b'Seivah Tovah" (he lived to be 70); Rashi there says that at 70, most of his hair is white. He also brings a source from Iyov.
What do we learn from "v'Im bi'Gevuros Shemonim Shanah"?
Mo'ed Katan 28a: One who dies at the age of 80, this is death of Gevuros.
Rashi: If his days increased powerfully, they are 80 years.
Radak: If a man's nature is strong, he will live 80 years. More than this is very rare.
Malbim: If our years will be mightier than usual, they will be 80.
Why does it say "v'Rahbam Amal va'Aven"?
Rashi: Al the grandeur and authority that a person has in these days is only sin. This is because "Ki Gaz Chish va'Na'ufah" (amidst anger, we 'fly' and die quickly).
Radak: "V'Rahbam" is a noun or Makor (like an infinitive). Even one whose days are serene, they are [like] sin, for they will not extend.
Mesilas Yesharim (Perek 1): This is why no intelligent person could believe that the purpose of man's creation was this world. What is his life in this world? Who is truly happy and serene in this world?!
Malbim: The pride of the days will be sin, i.e. the action in which he aggrandizes his days.
Why does it say "Ki Gaz Chish va'Na'ufah"?
Rashi: Refer to 90:10:3:1. "Gaz" is an expression of passing, like "Nagozu v'Avar" (Nachum 1:12), "va'Yagaz Salvim Min ha'Yam" (Bamidbar 11:31).
Radak: The strength of his days departs quickly, and he flies (goes quickly, like a bird) to the grave. Gaz is leaving one's place, like "va'Yagaz Salvim Min ha'Yam" (Bamidbar 11:31), "Atah Gozi" (above, 71:6), just those are Po'el Yotzei (act on others), and this is Po'el Omed (the action applies only to the subject).
Malbim: "Chish" is the flow of time; every moment is quickly cut off from existence (it ceases to be present, and become past). We 'fly' from existence to absence, for man's life is time. Every moment, a moment of time is cut off from his life, from existence to void.
