What do you mean by "designed to last x years?" Does "last" mean that the device should function precisely as it did on day one? And how long should "x" reasonably be? To what standards are Samsung and other cell phone vendors held? (I'm not arguing against your point—I don't know the answers.)
Having read Apple's statements, most of the MacRumors threads, and multiple comments on tech websites, I'm left with the following:
1. Li-ion batteries have unavoidable limitations: their performance degrades over time, and the decline may be accelerated by how they're used.
2. At this point we don't know if Apple was aware that iPhone batteries' capacity was too low to expect reasonable performance and life span in typical use cases. Personally, I'm in favor of larger batteries, even at the expense of thicker iPhones.
3. Apple's decision to limit energy draw to prevent shutdowns without informing users was wrong. The question is how do do this. Apple's message to customers says "iOS dynamically manages the maximum performance of some system components when needed to prevent a shutdown." If this is happening moment-to-moment, how would iOS let the user know? By flashing or changing the color of the battery icon? Or could they warn the user periodically if their device's battery state is such that this feature may need to kick in and let the user decide?
4. As a longtime Apple customer and shareholder, I don't believe that they were purposely degrading performance to drive upgrades, but I understand why it appears that way. Apple has been a very secretive company for many years; while I don't fault their need to protect their IP, their lack of transparency has hurt them, in this case by showing the company to be reactive rather than proactive.