There are many factors on whether to use a one handed or two handed backhand in tennis. It's subjective which one feels better, but there exists an objective answer to whether it makes a particular player better or not.

And just because it's hard and experts can get it wrong, doesn't mean expertise means nothing and "it's all subjective".

The fact that there exists a right and a wrong definitially means it's objective.

And for the business, or the project, there is a right and a wrong, on whether to build an OS or not. Even if it's very hard to predict, and not feasible to A/B test.