WINE is also a volunteer project, made by people in their spare time. They also had to reverse-engineer the entire Windows API, including its many, many quirks. Considering that, they've done pretty well.
Rather than re-implementing the same, 30+ year old, crufted together API, I'd rather see them design a new, modern OS with a WinAPI virtualization layer. Somewhat like what Apple did with OS X and the Classic environment.