To me, though I own two BMWs (which I discussed in detail above) a big driver is cost. I have a 2004 Expedition with 107,000 miles on it. It is in great shape and is paid for. I'd love to "upgrade" because the gas mileage is so dismal but the cash I'd spend on an "upgrade" would pay for enough gas, roughly, to carry me around the planet roughly 4 times. It doesn't make sense from an economic standpoint, or even an ecological one. The energy required to make a new vehicle from raw materials is absolutely enormous.
I really like the 2013 KTM 990 Baja at Midway Cycle in Madison, AL. Brand new bike. It's $14K. Subtracting the measly $3K I paid for the R100GS leaves an $11K difference. Imagine the R100 I could build putting say, $5K into the existing bike, which was exceedingly well taken care of and has only 28K miles. That leaves a difference of $6K. Would the 990 be $6K better? $6K would fund a pretty decent motorcycle outing somewhere. Clearly I think too much. Glad I have a really cute wife.