I believe that it is the other way around: The brushes
on a 24V starter rewound to be a 12 V starter would not last as long. The brushes on a 12V starter rewound to operate on 24V would last longer than it would have on 12V. The reason is that in both cases the starter must have the same peak power. For the same peak power the current
is half when operating on 24V and, therefore, the brushes are taking it easy, relatively speaking.
It is not uncommon to add an additional 24V alternator to an engine. AS was mentioned before one can change a regulator
to make a 12V alternator work at 24V or vice-versa. The stator magnetic saturation will not be optimum for a given rotor field winding current
maximum rating and so the power rating of the alternator will have to be derated some in both cases.