It sounds more like you're bringing out the strawman when you start talking about the average person who buys a computer at Wal Mart. We're not talking about them. We're talking about you. There's a plethora of competitive browsers in the market. You just happen to be using an OS that is not competitive in the market anymore.
The only developers who still see a return from going out of their way to support Windows XP are malware authors.
Last I checked, almost all applications I have on my XP is being supported by software developers (given that I don't have many). From the perspective of Office, all open-source alternatives to MS are supporting XP. Firefox is still supporting XP. Chrome and Opera does as well. So are you suggesting that they are breaking the security model by supporting XP. Why would they? I have yet to see any disclaimer from any of them that says that I should use their software on AS IS basis.
The only developers who still see a return from going out of their way to support Windows XP are malware authors.