Srsly? 15-20 year? I think Windows will be dead in 7-10 years. It shouldn't still be alive today. Windows is an OS for physical media. It wasn't built for networking and security. Windows'core needs to be scraped and MS needs to come up with something fresh from the bottom up.
Actually, they don't need to do anything. Others have already. MS just needs to bow out. You can't be on top forever.