[vlehto]

Wind turbines are currently over 100 meters high and installed into places that see lot of wind. It's a rule of thumb that you should keep the natural frequency of any large steel building above 20 hz. Swiching to heavier magnets will increase the mass of the generator which is mounted on top. That in turn will make the whole thing more wobbly (ie lower the frequency). To combat this you need thicker frame. Which in turn will cost more and weight more.

It's completely possible that current windmill designs are only possible with neodymium magnets. We would need to ask turbine designer.