I guess they'd rather charge their battery more often and accelerate the decline. The solution is just to replace the battery. Also, I don't think you can even update iOS on an iPhone 6S anymore. Seem odd to point out the battery as planned obsolescence when you can't even update it and apps will eventually end support.
Companies aren't going support products forever and even if they tried the devices naturally degrade from use. Caps wear out, thermal cycling takes it's toll etc.