Yeah, but there is evidence of Old English surviving into use by the 13th Century, long after the Normal conquest (discovered by no less than Tolkien himself hilariously), so the original population existed within most of England far past William I. After all, what is the sense in replacing the population when you can just rule them? Someone needs to work the farms. They killed a lot of people during rebellions to keep the new territory in check, but it really was the ruling class that changed for the most part. The Anglo-Saxons that fled to those various places were nobles and some soldiers, not the commoners. English didn't survive just because of speaking a common tongue, it survived because most of the populace never stopped speaking it as it evolved into Middle English. Henry V was the first English king to be a native speaker of English since Harold Godwinson, so it was Norman French that actually gradually disappeared.
Anyway, the point is actually that the people living in England did evolve, their culture did change and become something both based in what it was, but also new.
Hmmm, that is a massive simplification of the 13000 years and more of history that led to those nations becoming industrialised and conquering the world though. They are a direct product of a unique set of factors leading to farming starting in the fertile crescent, and progressing all the way through wars and migrations and culture evolution to result in those nations forming and having the technology that was ahead of the rest of the world. Guns, Germs and Steel covers the whole topic very succinctly, great book.
That warfare has actually been very important in the development of Eurasia and the dominance of Europe- you mentioned that China had potential it did not realise in the past and stagnated, and it was likely the lack of competition due to the political and geographical isolation of China at the time, as well as it's political unity, that allowed it to stagnate. If it had had rival states at it's borders as in Europe, then deliberately blocking the progression of technology as some of the Chinese governments did would have resulted in China being rapidly outstripped and probably conquered.