I study how humans can adapt genetically and strategically for long term development and survival. It seems to me that this issue should be advised with some biology. Often, the Great Replacement Theory (GRT) is just how nature acts. One population of a species just moves in on the territory of another population, replacing it. More often in though in the case of human, that has meant they just kill off the males and kept the females. The genes go on. If a population is replaced it is either likely the population that remains separate after hybridization or it was not very adaptive in the first place.

At the same time, human progress has been by the coming together of peoples. The hybrid is fitter and nature has consistently selected for it.

[From "Genetics For A New Human Ecology"]

To be most accurate, you need to think of a species as a section of a river of genes that is flowing through time.

Both genes and culture (memes) need to be thought of that way. They are tributary streams and rivers that come from different sources. They pool up along the river and go off into side streams and then may rejoin later. Some disappear as they slowly drip into the river or parts may even soak into the soil. Cultures disappear, but their genes have been mixed by migration, war, and slavery. They go on as part of human genetic wealth.

So survival wise, the best outcome for a race is to become part of the genetic river, then they can never be replaced. This is especially true during this time of great change. No one ethnic group is adapted to the unknown future humanity needs to adapt to. It will take all the genetic wealth and variability humanity has available.

I'll stop here and say that if you want a 100% different view on this and different than you have ever considered, may I recommend the book "Genetics For A New Human Ecology" which is more up to date than the YouTube video. It might change how you look at many things.