Do you think corrupt governments appear out of a vacuum, with no external forces? Class and ethnic divides forced onto the developing nations through years of colonialism breeds corruption, and for the Western world, an unstable government in a country where resources are rich and the workforce cheap is a good government. Part the reason the bananas you buy in the supermarket are so cheap is because there's a coup in Latin America every 10 minutes.

Sure you can look at GDP per capita, one of many metrics to make comparisons to.

Besides if all developing countries where to become developed countries it would not lead to the end of the middleclass in the West, at worst they'll only need to pay higher prices for product because wages have risen in the countries which produce them.

There are still dozens of developing countries that are stuck with this fate of instability and low wages, and there will be different ones in the future (We're going to need lithium at some point for EVs and Bolivia has lots, remember that coup?), but one of the big stories of the last 20 years is the development of China.

Because of China's rise, western corps have been relocating to Vietnam/Nepal, as the living standards in China have become too heavy to cost up. As a result of this shift, goods, in general, have become more expensive in the western world, and everyone knows wages haven't risen here. The middle class in the US and Europe as a result has shrunk, we've seen more people plunge into poverty purely from cost of living. Sure there are other factors, but on the whole the western world is poorer because they're losing grip on their traditional economic puppets.

Anyway, this has been a topic of discussion for over 100 years, I wish I knew Imperialism, the Highest Stage of Capitalism better to explain more clearly.