WI the Catholic Church hadn't adopted the Trinity?

[Philip]

A Christianity which was unitarian rather than trinitarian wouild presumably have had to abandon the notion that Jesus was the son of God

You presume incorrectly. Arius never questioned whether or not Chris was the Son of God. He did question whether or not Christ was eternally existent. Arius asserted that 'begotten' and 'created' should be considered synonymous -- the Christ was the first thing created by the Father. He questioned neither the identification of Christ as the Son of God nor Christ's unique existance and role in the Christian tradition.

Just like modern Unitarians, the church would probably evolve a more inclusive conept of God,

Modern Unitarians reflect modern ideals. Historical Unitarian movements usually did not follow this pattern. They were at least as exclusive as those they separated from.