http://wiki.mabinogiworld.com/index.php?title=Windmill
"Following the G14S2 Patch, Windmill's damage will be halved while in Deadly status."
Windmill-based melee combat was obsolete as it is, offering marginally less damage per hit for significantly more risk. It only dominated when there were low HP/high damage mobs with predictable aggro patterns. Without the ability to spam it at any time, and without the addition of further options to melee combat (other than Lance Mastery, and we don't know whether that's not a gimmick yet), there is absolutely no practical reason to specialize in melee.
The only way to avoid having your damage gimped after 10 WMs is:
1: have a high rank healer in the party
2: spam LOTS of HP pots.
So, barring this, how is melee supposed to deal consistent damage? To take advantage of their most powerful skill, they essentially have to consume potions like mages, except that mages deal more damage with greater distance. Or they can rely on Smash/combos and be dead weight in most parties.
Windmill needed to be buffed, not significantly nerfed.
(incidentally, this nerfs Golems FAR beyond what an appropriate nerf would be. Rank 6 WM for non magic ones is acceptable. Rank 6 for non magic ones PLUS being only able to WM 10 times at max before becoming relatively useless is not.)