Not good enough and a very short-sighted reply.
While I agree it's an obsolete device, I bet you wouldn't be saying the same if it were your MacBook. My one for example is an August 2013 MacBook Pro retina. Technically, that's more than 2 years old and it still runs everything perfectly and I expect it to do so for another 3-4 years. If it slowed to a crawl after 3-4 years because of the OS, I would be pretty pissed off with Apple.
Apple should simply block updates to older devices that simply cannot handle the demands of a new OS. At the end of the day iOS 8 is still perfectly capable and there is no real need for iOS 9 on the 4S. On top of that they were still selling the 4S a year ago through official channels so as a user, I would expect a minimum shelf life of at least 3-4 years from the point of purchase.
Incomparable to a MacBook. The mobile field is newer, and has just now gotten out of the "over 100% improvement year over year" phase.