How about adding a feature directly in kernel that send to a central common database a count.
Each running kernel would therefore send his count and we would know within a month how many running kernel around the world.
Ok, this is utopia because :
older kernel wouldn't have this option
what information exactly sending : count of processors ? uptime ? how many times is it used ? is it a server or a desktop ?
Last problem, and worst of all : who will ever let such a feature be included in the kernel ? I guess no-one. It's linux. Not Apple, Microsoft or Google !
But sometimes Big Brother would help...