I don’t think there’s any evidence that ‘moderate’ hand conditioning causes osteoarthritis (remember that’s caused by the loss/wearing away of cartilage lining the joints and bone rubbing against bone). Hitting a makiwara hard and often might deform bone or cause spurs etc limiting the range of motion of the delicate fingers: the hands are a marvel of evolutionary engineering by natural selection with the ‘intermediates’ viewable in our primate lineage.
The question is, why would you risk damaging your delicate hands in the modern age when there’s absolutely no need for it? I think it’s simply a macho tradition. Calloused fists are a badge of being tough, thwacking a makiwara, striped to the waist with a black obi wrapped around the waist, a romantic image. But there’s no need for it, if there ever was.