The very nature of this new world demands products are replaced rather than put on life support. Without the quick purchase turnaround time, the innovation cycle will slow and perhaps cause the fast growing movement to collapse altogether.
Yes, I just suggested that if Apple made the iPad user-serviceable, tablets could fail.
Innovation is fueled by profits. Without profits, companies cannot invest into research and development. If you do not buy a new tablet every several years, these profits will dwindle and cause innovation to slow. Companies like Apple do not make money on servicing products. Money is only made when they sell you a new product. For better or worse, this is the way of the consumer electronics world.