"Here in upstate NY, it isn't such a big deal. Except for certain
developments
of McMansions, house vintages are pretty mixed, and turnover is slower
than in
west or Texas where I grew up and my father sold residential real
estate. So
there's not so much looking to what the house values will be just 5
years down
the road... "

And one of the reasons turnover is slower and houses aren't
appreciating as fast is becaue ,amy people are choosing to live in
areas where they don't have to worry about the cheap inter-mixed homes
that lower property values. And those that don't expect to pay less
for any house in an area that's a hodgepodge jumble of nice homes and
run down ones.


"I'm talking small town and exurban upstate NY. But there are cases
like in NYC
outer boroughs and close-in suburbs, where this thing about housing
values has a
pretty nasty history of intentional racial block-busting. "

I think the racial block busting thing is more talk and urban myth than
reality. I'm sure it occurred long ago in some areas, but I don't
believe it was ever very common. Certainly not a factor now.


"In Manhattan, on the other hand, high-priced spaces exist quite
knowingly next to more run-down
spaces. "

Not true there either. High price is relative. If you look at areas
where all the properties are first class and expensive, eg park ave,
5th ave, the property values are considerably higher than if you look
at a similar building in an area not far away that has much lower price
buildings and/or tenants mixed in. You could still say the other
building is high price, but it's not as high as the building in the
nice, premier neighborhood, where you can't just build anything you
want.


"There's no *essential* reason why house sizes/prices can't mix. In
many places,
they do. "

No, I agree there isn't. Nor is there any reason areas that choose to
limit what kind and/or price range homes can built shouldn't be free to
do it so they can keep their area the way they want it to be.