Rhodesia was simply not sustainable. The white minority was too small to maintain control once the black majority had the means to rebel. They didn't exactly help themselves by alienating all possible allies either.
But the path Smith decided to take was of a total victor's peace where the Blacks had no choice but to kneel before him and take whatever scraps he deigned to throw them. So naturally, it turned into an All Or Nothing conflict where neither side was willing to be generous or conciliatory, and the White Minority was doomed to be on the losing side.
Yes I'm aware of the actual history, which is why I'm bringing up an alternative.