[SauronOfMordor]

how can increasingly poor people be customers for the factory-owners products? Isn't it in the capitalists interest to have customers able to buy what's being produced?

The name of the game is standard of living, not cash

If lots of people are poor, then it's really cheap to hire the army of gophers, servants, and assistants that really determines the lifestyle of the rich. Also, the elimination of the middle-class means that there will be no source of competition for the upper-class, so they relax. Imagine being a wealthy person in 1980 and having a chunk of your fortune invested in IBM, while unknown to you people like Bill Gates, Steve Jobs, and Mike Dell were tinkering in their garages.

A static environment favors wealthy investors who don't have the brainpower to spot new trends early.

[MrNatural]

I see your point; from the point of view of the capitalist, if they're making enough to be keep their wealth, and their income
is sufficient to hire the army of servants, and potential competion from below is hamstrung, then as far as they're
concerned, the system works fine.