I think when Musk says "Western Civilization", he's thinking about unbridled capitalism where increasingly rich people think they made all of their money themselves, and meanwhile, the poor people who they exploited and stole all of the money from end up living the lives of a slave.

Basically, almost everybody in the 1% has this sort of skewed view of the world, and all of their experiences seem to back that up. If you don't think that way, then you'll stop trying to earn money once you have enough. Almost all of Musk's billionaire peers are going to think the same way.

Then, he sees somebody like MacKenzie Scott, who didn't start out ultra wealthy, and who didn't really fit into that group. But because she was married to the sort of lunatic who belongs in that group, she ended up ultra wealthy. She likely doesn't share the mental problems that the rest of the group have. So, everything she does is going to seem completely incomprehensible to a person who completely lacks self-awareness like Musk.

She's just giving away money to people who would otherwise be much closer to the ultimate goal of the slave class. To a guy like Musk, that is the same thing as "the decline of Western civilization." I think all of the top billionaires share a mental illness, and we, as a democracy, have been remiss in our efforts to help them by setting limits on how much wealth they can accrue.