They would have that desire if all their tenants were suddenly wealthy enough to pay those rents, was my point. Which speaks to the broader point that these landlords are not working to keep these people in poverty.

All of those tenants suddenly becoming wealthy enough to pay higher rents is a magical hypothetical that undermines the basis of this discussion. Someone else on this thread said it succinctly:

"If this market were wealthier it wouldn't exist"

That is exactly right.

The landlord would be more likely to wait for them to move elsewhere and just await more customers for their current offering as long as the market was there. This, rather than outlay the capital to upgrade the place; else the landlord would have likelier invested in a more upscale offering to begin with.

Likewise, depending on the degree of increased wealth, the tenants may well prefer to move. Slums and trailer parks generally don't have a tendency to just "happen" in otherwise affluent areas. With increased options, renters would likely desire better surroundings, schools, ownership, etc. vs new granite countertops in their double-wides.

Of course for this same reason, landlords don't want to risk upgrading beyond what can be recovered in a reasonable time by rents that the market will bear. No amount of upgrades is going to make more affluent buyers with other options move to a mobile park.

People are suggesting here that the landlord would upgrade because it's economically rational to collect higher rent, while simultaneously ignoring much of the actual economic reality here.

Again, I'm not claiming that the landlords create the market. But they are under no delusion when they invest.