iPhone 6S available 25 September 2015, iPhone SE available 31 March 2016
Might not have been measurable, and might just cost an extra engineer without really harming anything.
If they had gone 64-bit-only, I'd be much more in agreement with you. But if you're still supporting a 32 bit platform, you have a lot of restrictions put on you, and the marginal cost of keeping support for a second 32 bit platform is not a big deal.
At minimum, slow systems require gracefully degrading GUI, which increases complexity by an order of magnitude. The graphics card capabilities are a much better first approximation than the word width.
If it took just a few extra engineers, or a few thousand, to fix the bugs that they have, Apple would have done it. The Mythical Man-Month, and all that.