Air-cooled motorcycle are simple and less complex, though. When Suzuki launched the GSX-R750 in 1985, it was oil/air-cooled but, in racing, the old engine was losing power as it got too hot. By the time the GSX-R750WN arrived, in 1992, its associated water jacket, plumbing and radiators were so much smaller, thanks to the onward march of progress, that the overall engine dimensions were slimmer than the previous GSX-R750M.
Of course, everyone else was on liquid-cooling anyway, and it was crucial because it meant that, as revs and heat increased, power didn't drop off.
We'll probably never see the end of air-cooled lumps, as they are still favored for naked motorcycle simplicity.