Planned obsolescence is more profitable.
Why should a company sell you a quality product that will work for 20 years with occasional upkeep instead of selling you 5 products over that same period of time that will kind of work, each ostensibly "better" than the last?
On the other hand, technology is growing at such a rapid pace that planned obsolescence is almost necessary. I certainly wouldn't want a 10+ year old computer to be my primary machine for recreational usage.