Progress != planned obsolescence. The iPhone 7 has more than an order of magnitude more compute power compared to the 4s, and app developers are targeting that device as it's actually selling today.
That alternative effectively means dropping support entirely (or allowing the whole ecosystem to stagnate based upon the lowest common denominator), something most android OEMs do after a year or two. You can't have it both ways. We're lucky that competition in this space means consumers have choice as to which style best suits their needs.
You can have it both ways. They can support the old version by providing security and stability patches without adding the new, computationally expensive features to the old phone.
They can at least allow downgrading to old iOS. I don't really understand their reasoning. I can decline upgrade request for years, but if I hit "OK" once, it's game over. Either upgrade automatically and don't ask me, if you're so security-consicous, or allow downgrading.