aqwsed12345 : Many scholars ... have extensively argued that the use of nomina sacra reflects the early Christian desire to show reverence for the sacred names of God and Christ.
These conclusions seem reasonable. Larry Hurtado, whom you mention, says regarding the nomina sacra, "at a point still earlier than any of the extant manuscripts, one of these four ['Jesus', 'Lord', 'Christ', or 'God'] may have been written in the special manner, from which the subsequent Christian scribal convention developed." (Lord Jesus Christ, 2005, p.626). That seems a more reasonable explanation for the evolution of this practice than to suggest that the original writers all started abbreviating these four names independently and without reference to each other, as slimboyfat points out above. Hurtado thinks it started with the name "Jesus", but it could just as easily (and in my opinion more likely) have started with replacing the tetragrammaton/Iao if the original writers followed the pattern of the first century LXX in their treatment of God's name.
My reason for stating that the original writers did not use the nomina sacra is, first of all, there is no record of nomina sacra being used in the LXX in (or before) the first century (when most scholars believe the majority of the NT was written). If we accept that nomina sacra were a Christian innovation then this is not surprising. But as they were all Jews they would have shared the Jewish view that scripture should not be altered, so especially when they were quoting from the Torah or other writings it would have been anathema to represent God's name in a way different to what they found written. It is more reasonable to suppose that when gentile scribes who did not share this view of sacred writings were tasked with copying, they took it on themselves (or were authorised by some sort of editorial board) to make these changes.
Your suggestion that the use of nomina sacra in John 1:1 for both references to God is a precedent for capitalising both in English is something I will have to give some thought to. However, as we have agreed that the second reference ("the Word was God") is qualitative, my inclination is that capitalising "God" blurs the meaning with that of the first reference ("the Word was with God") for English readers.