Well, this is your opinion, and not representative of every users experience. I upgraded from the iPhone 4 to a 5S for one reason only -- the home button and lock buttons had stopped working reliably. First one and then the other. Otherwise I was perfectly happy with the performance of my iP4 under iOS 7. And even with those hardware problems, I didn't upgrade until after the iPhone 6 was released. So only a little over a year ago. In fact, I still use my iP4 as an iPod Touch, despite the broken buttons, and am totally satisfied with its performance.
So maybe, the problem isn't planned obsolescence, but rather your needs have exceeded the abilities of the device, which is a totally different thing. Case in point, I wouldn't go back to an iPhone without Touch ID for any price now.
You may have noticed my screen name, and graphic. That's not just nostalgia -- I actually still have my original 128k Mac I bought used in the Spring of 1986, and I maintain and use it occasionally. And here's the thing -- it still does everything I bought it to do in 1986, and even a few things it was never designed to do thanks to industrious developers. What I have no expectations for it to do however, is something I need my current Macs to do.
But perhaps I misunderstand you -- if your complaint is solely that you wish you could go back to iOS 6, and that would make your iP4 totally usable to this day forestalling your need to upgrade to the 5S, but Apple won't let you; then I sympathize. There are ways around that however. But if you're saying you want the features afforded under iOS 7, or 8, and on the iPhone 5S along with its performance, then I don't really see this as planned obsolescence, but a failure of unreasonable expectations.
Again, I would still be using my 4 with no complaints, blissfully unaware of the convenience of Touch ID, holding out for the new 4" iPhone. Maybe you just had a bad install. But if that's not it, then I have to question the reasonableness of your expectations.