It would take a huge amount of time, effort and money to reinstall and reconfigure, say, 80% of all computers in a country that happen to be running Windows 7 o

It would take a huge amount of time, effort and money to reinstall and reconfigure, say, 80% of all computers in a country that happen to be running Windows 7 or newer. To recover all the data from backups (assuming the backup servers weren't running Windows as well). To replace any literally bricked hardware. Meanwhile, many very critical systems that happen to be implemented on Windows would shut down for weeks or months. Look at the current ransomware attack on the NHS. Now imagine taking down all health services, ISP services, maybe electricity or gas or water companies, supermarkets, distribution and shipping management, police and government records... Yes, these things usually have non-Windows components on big iron or Linux servers. But if you take out all the Windows clients, and however many Windows servers do exist, then the system as a whole cannot function. This could be a part of a (cyber)war effort. Instead of, or before, sending in the army to attack, you cripple the enemy's economy and civil infrastructure. You even have a chance of maintaining plausible deniability if you attack the MS update distribution channel, or serve them a sufficiently secret order that doesn't get leaked.