"It makes no sense from a marketing perspective because if one of these security holes is exploited do they really think those people effected are going to go to Windows 10?"
They probably do, but your point is valid. When people hear about a ransomware or other bit of malware that hits Windows 7 or 8.1, they're going to hear "Windows got hit with malware... again." The observation that it only hit certain versions of Windows is a distinction MS hopes they will make, but that is quite doubtful. It will just add to the "Windows" and "malware" terms being connected once again, and people will avoid 10 as much as the others-- and rightly so. If we can't trust MS to support their previous versions of Windows, we should not trust them to support 10 either.