This rumor has been around past few months, hoping it's true. Though while us technies in the minority would rather bug fixes than new features that don't apply for us, the general public would be very upset if iOS12 changes are all under the hood, as they would say Apple has fallen behind and people will insert common statements every years like they are finally considering switching etc. So while I hope the do fix…
The Mac has had an optimization-centric release following a feature release many times (all the Snow releases, El Capitan). I don't remember mass complaints. (By its name, High Sierra ought to be an optimization of Sierra also, but its bugginess suggests otherwise.)
Consumers and shareholders like cell phones because they're shiny.