You seem to forget that "slums" were not always slums. They used to be reasonably nice places to live. (Some of them in the US used to be really nice, posh places to live.) All it takes to turn a nice neighborhood into a slum is a lack of income to keep the houses maintained properly. Human habitats fall apart pretty quickly when they're abandoned or neglected.
Coming 2046: P2P food sharing.
That's a wild assumption. I would predict that it's more likely all that additional value will be captured by already-rich business owners and shareholders, not consumers.
If you are concerned about that, lobby to create laws that make it easy for consumers to invest their money into this kind of undertaking.
I don't think food and manufactured 'stuff' has ever been cheaper relative to median wages than it is now.
Wasteland/slum scenario seems unlikely; if 40-50% of all people did not live in a proper house[0], there would be a lot of empty houses.
(0) as opposed to a hut/shed