Wind has available energy because is has mass and is moving so we are trying to extract a bit of that kinetic enenery. Just for fun imagine what would happen, if at some point in time, you could extract it all. The air just past your mill would stop because you've extrated all it's power. Wind however is pretty smart. The next little bit of moving air in front of your mill would have two choices: smash through the stopped air your mill has just created while giving up all it's power to your blades(hard), or just go around your mill(easy). This is the reason why we have theoretical extraction limits.
You're talking about the Betz Limit, which has been discussed in detail here.
You have to slow down the wind to extract energy. But slowing down the wind reduces the amount of air through your mill to extract energy from. Extract all the energy and you get no more air to extract energy from. So there's some happy medium extraction percentage where you get the maximum total energy from a given swept cross-section of air. (Very much like the laffer curve for government income versus tax rates, where higher taxes slow down the economy, and a 100% tax rate stops it dead. B-) )
Betz identified the issue and calculated the maximum extractable fraction of wind energy. It's about 59.6%
From that you can calculate the amount you want to slow the air to transfer the right amount of energy to your blades to get the most from the swept area. And from that you calculate your blades. Vor a HWAT a little figuring gives you a "static airflow" model of how fast the blades are moving at each radius, what apparent wind they see, how much they deflect it, and the forces that result.
A high TSR (tip speed ratio) HAWT (horizontal axis wind turbine, i.e. a "prop on a stick with a tail") can easily get within a couple percent of the Betz Limit, and the designs have been proven in wind tunnels. The guys here, near as I can tell, are using the design rules to build exactly such blades.
(VAWTs (vertical axis ...) don't do as well - like they fall off from the ideal by maybe a third or worse, depending on type. But they have other advantages - like working well in turbulent areas and being easy to make BIG, to compensate for the reduced efficiency. Nevertheless, the HWAT designs work so well in most places where you have enough open land to raise a turbine, are so simple to make and raise into faster wind, and so easy to design to automatically protect themselves from high wind, that they end up being the big winners here.)
Point being, though, that a little analysis and simplification eliminates the need to deal with the complex tradeoffs you're concerned about in a single lump. Once you know where the sweet spot is, you just go for it and ignore the rest, then deal with the situations when you can't hit it by other simple rules. Analysis lets you resolve the mass of complexity into a combination of a small set of much simpler pieces.
When you have a complex problem, split it into simple problems and solve those. Fundamental design principle.
Of course when you walk into a design discussion after that analysis is done, you'll just see the consideration of the simple components. This can look like the complex problem hasn't been addressed.
