"Windows I/O and scheduling could be tuned specifically for SQL Server, or just incorporated into the SQL Server platform."

I'm afraid if you incorporated these into SQL Server you would quickly have the functionality diverge from the base OS source, which could in the long run lead to strange consequences. I've seen code which was copied from a legacy system mutate so fast in the space of the year that the original designer could not figure out why it would no longer work in the new system.