Why will a huge portion of people not upgrade from XP? It's been at long time since you could buy a computer with it preinstalled (unless you specifically ask for it). Hardware attrition, upgrade cycles, and hopefully the end of the global recession should clear out those machines.
Any computer that came with XP preinstalled that is still around 15 years later is going to be so slow that an outdated browser is going to be the least of its worries.
Not so long as you think… Microsoft licensed XP for free on netbooks to compete with Linux (netbooks cost maybe twice as much as a Windows license). Summer of 2009, I walked into a Verizon store and they were selling netbooks that, swear to God, had IE6 as the default browser. In 2009!
XP installation packages, even updated through the last service pack (SP3), only ever came with IE6. You have to actively download and install IE 7 or 8. Not too surprising if the netbook vendors trying to compete on rock-bottom price didn't want to pay an installation monkey to do that. If you don't know what a browser version is, you're not a lost sale. If you do, you know you can easily upgrade it and you're still not a lost sale.
Is that a fact? I think I installed XP with IE7 straight off the disk once. Maybe it was slip-streamed. Hardly seems like a lot of work for the netbook OEMs to create a slipstreamed install with IE7/8.
Any computer that came with XP preinstalled that is still around 15 years later is going to be so slow that an outdated browser is going to be the least of its worries.