set affinity ... also in XP ,doesn't mean that software written for one core will use multiple cores to do tasks. Especially relevant for CPU heavy applications.

XP mode is a disadvantage in Win7. IT requires 2 os running , 2x memory footprint (limited to 4GB max on 32bit), if online ..2 firewalls etc.

I have NO software that will only run on Win7 and not in XP.
But I have lot's of software and some hardware that won't run in Win7 , unless I use XP mode , which I said uses twice the (limited) resources because it is two OS running at the same time.

fact is... 32bit / XP software is the majority.

Microsoft need to focus on making an OS that adds features for existing software.
Access to more ram , more processing features like enabling multicore support for single core applications.
NOT.. Release a completely new OS that is incompatible with the majority of software.
Where the work around is to use twice the system resources for the majority of software.

I have XP 32bit Audio software that loads instruments with rom data into ram , I have 4GB of ram filled in no time. If I use Win7 the ram I have to use is much less than 4GB. (ram is cheap... 32bit os with 64GB of ram / / why not ? it's 100% possible)
Some of my software is single threaded and very CPU heavy (maxes out the latest cpus because it only uses one core ), An OS feature that enables (forces) software to utilize all cores is needed.

I am not alone , the majority of business and corporations have custom software that will only work in Win7 if they update their hardware to 2X the amount they need. Even then they need to migrate parts of systems which is an enormous task.
It's cheaper and easier to continue using XP even after microsoft stop supporting it.
Most businesses will just use a software dev to modify the os.
After all , they have probably spent the last 10 years and enormous amounts of money creating a software system that uses XP as its skeleton.