You have to realize that a lot of (maybe "most") Packer fans have been fans for less than 15 years. That means Brett is the only Green Bay QB they've ever known. For many, he's the reason they became fans in the first place.
Their reactions are unfortunate and irrational, and their antagonism toward Aaron is certainly misguided, but it's a little more understandable if you look at it from their point of view. Brett Favre is the only reason for being for a whole generation of Packer fans.
Some of them will follow him to New York. Some will lose all interest in football. A very few will hang around Green Bay just out of spite. But their spleens will soon be vented, and they will find some other icon to cling to.
The bottom line is that, soon enough, the people who were Favre-firsters (as much as I hate that label) will be gone. And not missed.