On the sunny side of the street - the problem with monocultures

Three years ago three large white ash trees lined the street in front of my house and my neighbors spaced about 60 feet apart (all over 3 foot diameter trunks).  The city just removed the last one; it was clearly moribund.  Thank you emerald ash borer.  Three really big trees make a lot of shade and when you remove all three essentially at once you notice the difference.  The lesson here is simple: monocultures don't work.  They are inherently unstable and unworkable ecologically.  Someone about 70 years ago decided to plant a monoculture of white ash trees down our side of the street.  They were getting old, but you expect as they age, one by one, for them to be replaced, and someone chose white ash trees.  No imagination.  The newest trees are more diverse, red oaks, white oaks, new elms, and some smaller ornamentals.  The problem with monocultures is when one is susceptable to something, they all are, and they all die at once, and at about the same stage of life rather than being staggered.  There are virtually no exceptions, and yet is this not the most common of human cultivational strategies?  Our landscaping in part reflects the sun-shade environment as it was a decade ago with healthy shade trees.  A few of our beds suffered in last summer's heat and drought made worse by they more open exposure.  Fortunately our city arborist is being more proactive diversifying plantings and planning ahead and getting replacements going a few years before trees in decline are removed.  That's the way to do it.