Even if it was the best game in the world, how would this make the platform any more viable?
It's a chicken or the egg thing. Mac users who are gamers will have consoles/gaming PCs. The titles that do come to MacOS often come delayed behind the PC release and/or are a suboptimal port. So even if you wanted to account for that stat (ie. a Mac 'purchase'), it's not likely to happen.
A very good exclusive really opens up what the actual market could be if large studios want to target it. You get core gamers + the latent market of those who aren't serious enough gamers to buy dedicated hardware (aside from maybe a Switch) but may buy major releases simply because.