I keep seeing a lot of talk about huge numbers of cores as a solution, but I'm a skeptic in that regard. We've seen that in the Intel world, but it really hasn't yielded massive performance gains and at best just minor gains. So much software still struggles to use more than a handful of cores, and many computing tasks are not inherently capable of being parallelized--at least not easily.
I'm with those that think that this will be a laptop-style solution first and foremost for Apple. They'll have all native versions of all of their applications, and they'll make it as easy as possible for developers to make native versions of their own apps. Unlike the PPC transition this does *not* have to be a hard and fast break to ARM, and Apple won't care if it takes years. Their ability to already deliver multiple versions of their products means it won't be a big deal for them to still have x86 MacOS and MacOS apps for a few years to come while they snap up a larger segment of the laptop/tablet market with a machine that is uber slim, quiet, and has great battery life.
As others have pointed out, the vast majority of users are plenty happy to use a pretty slim set of applications and don't need a lot of power. A small ARM MacBook/MBA would be perfect for those folks. I could see Apple even making it clear that it won't run all of your regular x86 MacOS apps, but it will run iOS and iPadOS apps, and faster than on those devices.
One thing that seems to be overlooked here is that the overall computer market--both for PC's and laptops--has been shrinking for years now and will continue to shrink. People have moved to their mobile devices and tablets and that doesn't seem like a trend that will ever change. Apple leveraging the existing technology they have there would make sense as the margins on these devices get smaller and smaller--but it's a delicate calculus of how much they have invest to get to the point where the pro machines need to be in terms of power vs. the revenue they would get from selling them. It seems just as likely that Apple will drop out of the high-end desktop market at some point in the future as they would invest in a specialty chip and solution set just for that market.
Of course all of this is just speculation until we hear more in a few weeks.