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.