Planned obsolescence is a very real thing, but that’s not the reason your iPhone slows down after a few years - if Apple really wanted to make your phone obsolete, they’d just stop providing software updates for you, just like all the Android OEMs do.

Your iPhone only lasts about 4 or 5 years at most, because by that time, CPU architecture and software has evolved to the point that your 4 year-old CPU no longer cuts the mustard. As new iPhones are released and Apple’s SoCs are refined, the software is tweaked to take advantage of as much of the new horsepower as possible - but unfortunately this also means old SoCs suffer as they don’t have the same level of throughput as the latest iterations.

FWIW, I’ve been using ios11 on my 6s since DB1 and it still runs absolutely perfectly. Since iOS 11 is 64-bit only, it seems to run slightly better on supported devices compared to previous IOS versions.

TL;DR: it’s not planned obsolescence, it’s Moore’s Law - technology is evolving so quickly that technology from today will be completely obsolete within 5 years - the maximum term for IOS update support for IPhones. Apple don’t design your phone to fail, they support it until it’s simply too old/weak to run the latest software which is designed to work on the latest hardware.