I use the Fun Factor rationalization too, but I find it can be problematic in many cases. Mainly with bigger purchases. When you amortize that $2000 bike out per use, the difference between it and the $1500 bike can seem rather insignificant. "$1.5 v. $2 per use? Hell, I might as well get the better one." But, it's still $500. That's real money that could be used to earn more money somewhere else. Ratchet that up if we're talking about cars or boats or houses. So, I like the Fun Factor rationalization, but it can be dangerous.

As to the "lasting forever bit", I don't think it's necessary that things last literally forever; the point is more that making very long term purchases is a worthy goal. 15-20 years maybe. That said, I do try to buy things that literally will last me forever, though the list isn't very long, yet:

I try to buy tools that will last forever, or close to. With things like digital cameras (or any consumer electronics), it's silly to expect anything to last more than 10 years, on the very long end.

But, with power tools, I buy things expecting them to last forever (with routine maintenance and not heavy duty use). I more or less expect my nice knives to last forever. I expect most of my furniture to last forever. I expect my weight set to last forever.