Considerably longer? I don’t know about that. If you are talking the cheapie PC’s, then yeah, but for the mid-range and up they have as long a lifetime as Mac does, depending on what you do with it. That would go for Mac’s as well. If you are a graphics designer or some other power user then you effective lifetime for your PC is going to pretty short compared to other users as you will always want to have the best available components to power the software that will eat up every resource you can throw at it. However if you are the average user who does nothing more than websurfing and the occasional word document then the lifetime of your pc is as long as your hardware will ast, but too often consumers don’t see it that way.
Only if you have very limited expectations for Web surfing. Computers more than a few years old are going to have a hard time with YouTube, and with a lot of Flash pages.
The Web has shifted the definition of obsolescence; it's true that whatever a computer can do the day you buy it, it will continue to do for years and years; but long before the hardware fails, "doing the same stuff" will no longer include visiting the same Web sites.