Spacing of machines is a pretty tricky balance and if the wind patterns start changing in unpredictable ways that will be a very hard thing to design for. You may end up playing it safe and under-powering the windfarm or go for maximum power generation and end up with a number of machines switched off (or worse case: damaged) for longer periods of time. Steady windspeed (narrow distribution) is the biggest single factor in wind installation efficiency.
That's why some of the earliest wind parks were set up where there was a constant source of wind with a relatively constant speed.
This is a beautiful example of one of those:
The simplicity and efficiency of that design was only possible because of the wind conditions. The black blades are an anti-icing measure.
It sounds like you do have confident experience of modern wind farm specification and design.
I read that wind turbines are commonly built now with relatively over sized blades because after decades of development designers have greater confidence in their gust survivability and control mechanisms, and larger blades raise cut in speeds making better use of the most expensive part - the generator.
With your experience in these matters, can you say what percentage of modern wind turbine installations have been seriously damaged by being under specified to unpredictable wind conditions ? And very roughly what level of increased damage we might be talking about here from potential change in wind patterns.
That is not necessarily true, it depends on the allowed degree of pitch and the shape of the first 1/3rd of the blade which tends to generate the torque required to get the machine going because it is generally coarser pitched than the rest of the blade.
Once it is running you can just about ignore that first 1/3rd or even the first 2/3rds of the blade, the bulk of the power is generated by the outer 1/3rd.
> what percentage of modern wind turbine installations have been seriously damaged by being under specified to unpredictable wind conditions ?
Very few turbines have seen damage from overspeed or direct wind damage to the nacelle or the tower, for now this is a non-issue but if there is a structural change in wind dynamics then for sure there will be a price to pay, either in terms of machines ending up shut down more frequently, longevity issues or breakdowns.
> And very roughly what level of increased damage we might be talking about here from potential change in wind patterns.
That all depends on the magnitude of the change.
Suffice to say that wind power is generally installed for 20-30 year life-span of the Turbines, after that they become un-economical to operate both due to increased maintenance costs, wear on the main structural components and the blades themselves (the bending stresses eventually weaken the blades) as well as technical developments making the more modern machines much more economical to operate than aging ones. So as long as the changes do not happen on a timescalle smaller than that the effects will likely be limited, if they happen (much) faster then there could be a real problem.
Gusty and turbulent airflow has a further detrimental effect on lifespan.
I think you are correcting a different matter here. Blade diameter in practice dominates the amount of power a which turbine can be designed to generate at lower windspeeds, aerodynamic design choices have to work with whatever diameter is provided. Increasing diameter involves significant expense and the entire reason it is payed is to raise cut in / raise generation at lower speeds / increase capacity factor.
> Suffice to say that wind power is generally installed for 20-30 year life-span
Not in recent years. 15 year contracts for windfarms are the current norm. I've read this often while following industry news and it can be observed from querying the different times 
> after that they become un-economical to operate both due to increased maintenance costs
Its not known to be the case that they'll become uneconomical. It may just be more economical to upgrade them, there is uncertainty involved which is the manufacturers bet.
Its a great commercial advantage of wind farms (and solar) that they can be relatively quickly online and make a profit on investment in 15 years rather than seeking 30 or 40 or 50 year supply contracts as some competing technologies do because of higher construction costs.