LightSpectra
me autem minui
When half the world's population are Abrahamists, I don't think that monotheism is going away any time soon.
If by "Abrahamist" you mean "worshiper of the one God of Abraham" (i.e. Judaism, Nicene Christianity, Islam, Baha'i, and other faiths), then half the world is probably not Abrahamist. This is a problem that's actually quite severe in political dialectic, and I've considered making a thread on this in the Tavern a couple times, but it's a daunting work.
We have to distinguish between one's credo (what they believe in their innermost thoughts), their confession (what they publicly say their faith is), and their ethnicity (which is based on one's identity in a community and almost completely disregards credo). Why is this important? Well, we have the following cases: (1) people that are labeled as monotheists because they were entered into their particular faith as children, but as adults apostatized (e.g. a man baptized in a Lutheran parish, who is counted as a tenant because of his baptismal certificate but is a confessed atheist); (2) people who self-identify as a certain faith because they live in a community where it is dangerous not to homogenize, but nevertheless have nontheistic credo (e.g. a secular atheist in Iran who attends a Shi'a mosque on Fridays for his own safety); and (3) one who identifies with an ethnic group that is nominally of a certain faith but whose credo differs (e.g. an Israeli Jewish woman that confesses to be an Orthodox Jew for reasons that are political, nationalistic and communal, but whose actual confession is a complex mish-mash of Judaism, Buddhism and deism).
Once you filter out those three exceptions from the counted number of "Abrahamists" in the world, the number could dip into well below a billion, possibly under half a billion.