A literally post-scarcity environment is not a problem because it makes the price of everything zero. Food is free, shelter is free, medical care is free, you don't need a job. Do whatever you want all day, it's not a problem.
The problem is that what we're entering is not a post-scarcity environment, it's an environment where things many people don't already have, like energy or land, become more scarce and thus more valuable than the things they do have, like the ability to perform unskilled labor.
In theory capitalism works fine in that situation. If people can't afford rent or home ownership then the price of housing should come down, because the supply of housing and the number of people who need it haven't really changed, so if people have less money then landlords and home sellers can no longer charge as much.
The problem is that we interfere with that process -- the housing crash was a market correction. Instead of letting that correction happen all at once, we bailed out the banks. So now the banks that didn't file for bankruptcy and liquidate their assets are sitting on thousands and thousands of foreclosed homes and intentionally keeping them off the market in order to backstop housing prices and prevent even more foreclosures. But the result is a significant artificial inflation of housing costs that comes directly out of the pockets of people in poverty.