why is the population of european countries are declining and why it's not about the great replacement theory

 first of all if it was a great replacement about the whole europe continent, then countries such as china, japan, south korea would not have their population decline, even countries from south east asia such as thailand are also facing population decline, the philippines is also likely to face one. it's about the country being developed or not, not about the theory of great replacement, 

the more developed the country is = the less likely people to have more child

the less developed the country is = the more likely people to have more child

see it's about the country being developed, not the great replacement theory. even if you deport all immigrants, the population will still likely to decline.