Apple will also run into the diminishing returns, but they will retain the real killer advantage over general purpose CPU vendors that the have in other hardware areas: being able to retire or rework old or misconceived parts of the architecture entirely in future versions unilaterally. If they want to drop M1 support in some future MacOS version, all it will take will be a WWDC announcement that the next version won't simply work on that generation it earlier of machines.
Yeah, that all makes senses. But having legacy software that you support has been a big advantage for x86 for, basically, forever. For a lot of purposes, that is way more important than performance per watt.
