Britain couldn't, at the time, afford to lose any of it's shipbuilding capacity, hence the betrayal of the treaty, and Britain's unilateral partitioning of the island of Ireland. To suggest that because Britain had other shipbuilding capacity, it didn't need the shipbuilding capacity it had in Ireland, defies the historical record of events at that time in Ireland and England.

And the argument that civil war between unionists and nationalists would have been worse than the civil war between nationalists, is pure speculation which could never be proved. But it does make for a convenient excuse, used by British apologists and realm defenders like yourself, there Teribus, for Britain's unilateral partitioning of the island. That same British partitioning tactic was used throughout the Anglo and American empires throughout the 20th c. to ill effect everywhere it was done.