Global population is an extraordinarily poor thing to measure against! First of all that doesn't factor in obviously critical factors like age demographics, economic development, trends (inflation, average income levels), etc. But mostly its down to lifestyle and consumption.
When smartphones exploded in popularity there was a huge surge in demand because these are personal devices, whereas a computer or console is able and likely to support multiple individual users per hardware unit. These products were traditionally aimed at families as opposed to individuals like TV's and stereo systems.
Obviously most people in the developed world own smartphones because its borderline impossible to live a "normal" life without one anymore. Now if you consider that a viable alternative to owning a console for light gaming usage, its pretty obvious why install-base sizes of such devices have been stable for so long.
As anyone who actually LIVES in the real-world should realize, the market's basically been saturated for years.
The good news is that the population* is aging, and once gaming has become normalized as a regular form of recreation the market isn't likely to go away either.
*Again in the DEVELOPED world, the primary markets for these products.
The slowdown in the console gaming market has very little to do with mobile and much more to do with price inelasticity.
The semiconductor industry has reached a peak in process technology improvements that halve chip costs every two years, i.e. Moore's Law is dead.
Also, TSMC now having no competition from Global Foundries, Samsung or even Intel at the bleeding edge nodes, is now a monopoly and instead of pricing the latest most advanced process nodes more competitively, they're pricing to maximize profits, abusing their market position (because, fuck it, they can).
There are also other considerations, like EUV requiring ever increasing numbers of process steps, ballooning engineering and fabrication costs which counteracts any cost benefits smaller die sizes typically have on the latest most advanced nodes.
As such, die shrinks are no longer economical, so console have far less opportunity to drive down costs on the back end to access the more price conscious consumers in the primary markets as well as entire markets worth of secondary users in the developing world.
So unless, some new paradigm shift in transistor technology like graphene, spintronics, or photon-based chips can swoop in and reset Moore's Law, the future of affordable personal computing hardware for gaming is pretty bleak.