For all the similarities, macOS and iOS are different. Apple has OS X's Carbon 32-bit framework which they still maintain but don't enhance ( much ) and AFAIK, iOS doesn't have a separate 32-bit framework. While Apple could eliminate 32-bit Carbon from OS X, there is nothing compelling them to do so.
Apple doesn’t maintain Carbon anymore. It has been declared deprecated since Mountain Lion (2012) and the version has not changed since then. What you have in Sierra is what you had in Mountain Lion. Developers knew that this would happen since Snow Leopard.
Ultimately, it is a question of policy: do we keep old, unmaintained code around and ignore any bugs in them? Do we want to support legacy software forever and continue to saddle all customers with unnecessary code? Universal binaries and old frameworks come at a cost: they cost disk space. Abandoned frameworks may contain bugs and vulnerabilities. I think it is sensible to remove such code after the decision has been made. The more pertinent question is: should Apple choose to deprecate technology so quickly?