Of course demographics have an impact because it reduces the amount of youngster that may eventually choose to pursuit a football career but that can't be a reason (or excuse) to open the border to more immigrants. Otherwise how come small countries with very little immigration like Croatia and Uruguay are so good and they only have a few million citizens?
Let me share my perspective about this matter, where football is considered as best sport and gives the opportunity to make money for a player then where more people will be interested in doing it. You said why India and China can't able to find 11 good players is because in India cricket is where money rains so every youngster from their childhood dream of becoming a cricketer than a football or javelin throw player but ofcourse India is also now focusing on improving their football participation by replicating something called ISL after the success of IPL (for cricket).
India has almost 1.5 billion people, even if not everyone is interested in football, just based on numbers, they should have at least some decent players and yet they even lost against countries like Afghanistan that have been in war for years. Even North Korea managed to qualify for a World Cup before India. Of course they can't compete with Australia, Japan or South Korea, the problem is that they should be able to compete at least with countries like Afghanistan, Kuwait, Syria, etc.
Let me share my perspective about this matter, where football is considered as best sport and gives the opportunity to make money for a player then where more people will be interested in doing it. You said why India and China can't able to find 11 good players is because in India cricket is where money rains so every youngster from their childhood dream of becoming a cricketer than a football or javelin throw player but ofcourse India is also now focusing on improving their football participation by replicating something called ISL after the success of IPL (for cricket).
Every country has programs to improve its sport. Even though China failed to qualify for the 2026 World Cup, it doesn't mean they don't care about their football. It's just that Indonesia might have been luckier in the previous round, managing to beat China. As far as I know, at the Asian level, China's sport (in general, nit just football) was impoved, which means there are programs to improve football as well. However, Japan and Australia aren't weak teams that China can easily defeat, so that might be another reason why China failed to qualify.
China constantly failed to qualify to the World Cup since 2002, the news would be if they can qualify, not qualifying is the norm at this point. The problem with China is that they are very good with individual sports but the suck with team sports.