The only factor that is of any importantance in this scenario is the consumer. Whether we like it or not, and more importantly, if its what we want or not.
Yes, but if the consumer is complaining about performance when they've got say a 6 core processor in 32 bit(its bound to happen by the time Windows Seven is out, more than likely), yet with 64 bit the performance increase will be greater due to the kernel being able to fully take advantage of all 6 cores. Wouldn't it be in the consumers best interest to move foward to make it a BETTER product?
People are always going to complain, regardless of what is or isn't done. If Microsoft goes x64 only then people will complain who have computers that don't support it, yet the people that do will see the performance increase especially on processors with 4 cores and up, so now you've stopped one complaint and its been replaced by another. Not gonna even get into the memory restrictions of x86.