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What were their reasons for planting another monoculture, after the last one ended so badly? It seems obvious to me that an urban mixed forest would be more resilient against disease -- so I'm guessing that urban planners had some reason I'm not thinking of...

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Dutch elm disease (and emerald ash borer) wiped out natural tree populations in forests just as thoroughly as those planted in the city. While monocultures are not helping, that is a footnote to the real problem here: these diseases are invasive and native tree species have no resistance to them.

There is still elm growing wild. They are spaced out enough that transmission doesn't happen fast.

The geographic spread of the ash borer is less complete than Dutch elm disease, so it's less clear what will happen over the long term. Maybe pockets of saplings will outlast the insects.

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