Microsoft deserves ALL the blame for people still running XP. They broke so many hardware drivers with the XP->Vista change that people basically got stuck forever.
Then, not having learned their lesson, they pulled similar crap with the Windows 7->Windows 8 transition which pissed people off so badly that they refused to go to Windows 10 and are currently suing Microsoft for attempting to shove it down people's throats.
Insecure old version of Windows are Microsoft's own damn fault.
Drivers still run inside kernel. What was changed in Vista is that the kernel tries to detect when kernel data structures that should be immutable are changed, which some drivers do.
The idea of non-privileged drivers is neat, but in general is not worthwhile because the driver has to somehow access the hardware, which for significant amount of device/platform combinations leads to access to arbitrary memory locations.
Edit: perfect example are GPU drivers, which are for a long time typically composed of small priviledged kernel driver and all the complex logic in userspace. In many cases the interface between these two components could be abused to get code execution in kernel context (in 2k/xp times there was even RCE in kernel context triggered by displaying properly crafted image in IE)
As I recall, the issue they meant to address wasn't security but stability. Apparently a majority of BSODs were caused by faults in the driver taking down the kernel with it.
This means you aren't preventing drivers from having full access. You just need to prevent more unintended side-effects.
And you can do things like give people specific permission to access the kernel when doing the driver install. ie. "This driver does not adhere to Vista driver standards. Do you wish to install as an XP driver?"
Suddenly, people can run that business critical, single, old driver as unsafe while running the other drivers safely.
Alas, some manager at Microsoft decided it was more important to get his numbers up this quarter so he could get his bonus. In so doing, Microsoft orphaned a bunch of people on XP just like they orphaned a bunch of people on VB6.
Microsoft made its own bed; now it has to lie in it.
Then, not having learned their lesson, they pulled similar crap with the Windows 7->Windows 8 transition which pissed people off so badly that they refused to go to Windows 10 and are currently suing Microsoft for attempting to shove it down people's throats.
Insecure old version of Windows are Microsoft's own damn fault.