I have explained why God is necessary, I have explained why the universe is contingent.
Replacing one term with the other does not make it so, as God is not made up of parts but the universe is, e.g: the universe is made up of time, space and matter:
Time
T = ∑ [n = 1 to N] Δtₙ
Space
𝐗 = ∑ [n = 1 to N] Δ𝐱ₙ
Mass
M = ∑ [n = 1 to N] Δmₙ
The universe is the set of all three:
U ≡ {t, x, m}
Anything existing in U is contingent, i,.e., could fail to exist. God, however, exists outside of U:
G ∉ U
Therefore, God is not contingent, but U is contingent. Which is why it is fallacious to use the definition of God as the definition of the universe, as they are ontologically different things.
God is contingent, not necessary. If god were necessary then it would have always existed, other wise if there was a state where god did not exist then it would mean that for that state god failed to exist, i.e., contingent.