It’s an interesting subject and one I suspect where the data might be considered proprietary information by most people‘s employers. The internet is full of people suggesting you can fly or upgrade for free using various points/tricks etc.
Certainly my impression on recent long haul flights is that the business seats were all taken. There is no doubt that a number of people like airline employees and frequent flyers are getting to sit up front. But the majority over the course of the year certainly at peak times have paid for their seat, although what they paid is down to the yield management system. There are vast differences between at the simplest level higher fares in return for flexibility to the barely visible differences resulting from where you bought your ticket e.g. airline partners/country of departure etc.
I suspect the percentage of full fare paying pax in first is less than in business which is in turn less than premium eco. Be interesting to know what you come up with and how cooperative airlines are in providing data.