Some forms of monotheism might not be unjustified even if Theism is unjustified.
Here is the example of Christianity that might not be affected even if disbelief in Theism is justified. Jesus said, "The kingdom of God is within you" (Luke 7:21). Carl Jung held a view like this, although it wasn't expressly Christian (https://thesethingsinside.wordpress.com/2013/01/07/carl-jung-says-god-is-reality-itself/). He held that God is the collective consciousness that resides in all of us together. These are two of the most important religious thinkers in the history of Western civilization.
By contrast, in classical Theism, God, if He exists, stands external to a human's consciousness. According to Cudworth, the originator of the term theism, "Strictly and properly called Theists [are those] who affirm, that a perfectly conscious understanding being, or mind, existing of itself from eternity, was the cause of all other things" (Cudworth, Ralph (1678). The True Intellectual System of the Universe, Vol. I. New York: Gould & Newman, 1837, p. 267). Note well "existing of itself from eternity," an assumption not held by Jung or Jesus of Nazareth.
So disbelief in an externally existing God does not necessarily transfer to a brand of monotheism where God exists internally. I am not saying that disbelief in Theism is not justified (or giving reasons for or against the justification of Theism or the Jung/Jesus variety of monotheism); rather, I am saying that justification in disbelief in Theism only has as its object a very specific theology that is not universally shared by monotheists, which I have proven.
Thus, even if you were to argue that Theism is unjustified, you would still have work to do to prove that all forms of monotheism are unjustified.