It's really no different than what Apple and Google have done also, if M$ decides to play with open source....so?
Well, yes now we got .NET Framework, Apples Swift and other as open source. I think the difference here is what MS really makes with this, because it's open not means that it's integrated in the OS because without verification it's difficult to say/check. But besides this, the topic is more about the Malware possibility which have nothing much to do with open source or not. The protection mechanism are present and every developer can decide to use it and adapt it in his source or not. The problem is that not every developer care much about that. I still see software without ALSR (I mean come on it's simply a compiler flag you need). So the question isn't about complaining about the OS like in the 90's if all was 'new' and no one really know what it's coming, now we learned about such things and got several things we could use, but people still preferring to install an external AV without really knowing what it makes in the background or that it possible interference in your connection (because they possible creating an VPN tunnel, installing additional certificates and and and). Of course the malware guys are also smart and will migrate this and will find a way to bypass this, but as it's already mentioned Windows is now 'as servicer' which means we getting more and more updates, faster ones, people can test and report and MS directly response and reacts on this. This is how Linux worked for years now, I know but I think that Linux or other OS suffers from other 'important' problems, like community fights between the developers, too many distros which not have any benefit except that they are mostly fast outdated, the fights between hardware manufacture and many more. Windows is not really affected by this because they mostly working close together with them instead of fight e.g. against nVidia (like Torvals in the past).