I fail to understand why some white people cheer their own replacement.

Indoctrination.

I grew up in a very liberal part of California, surrounded by liberals/progressives. It was the status quo. I wasn't extreme anti-white, thankfully, but I did have negative views of white people, as a white person myself.

If you're taught from a young age - even passively via osmosis - that white people are bad, you're likely to believe it.

It also helps massively that saying white people aren't bad is verboten, so you don't hear the counterpoint much and, when you do, it's demonized, which just proves the point to the indoctrinated party. If the only people saying white people aren't bad are "Nazis" (yes, it's circular and self-referential, because they're Nazis because they said white people aren't bad)...well, white people must be really bad, right? The only people defending them are Bad People!

It's a really effective attack.