Well, plus Apple's M chips thus far, while very, very impressive in terms of performance at any particular power-point, are no where *close* to the ultimate power of AMD or Intel based desktop machines with desktop GPUs. They just aren't even in the general ballpark. It's a totally different game. It may take Apple many years to equal that performance level, or maybe they never will, who knows at this point for sure?
As an example, my 2009 MacPro 4,1->5,1 with the 12-core 3.46Ghz upgrade in place and an AMD RX 580 8GB nets a Cinebench R23 CPU Multi Core of 8444 and a Geekbench Metal Score of 47855. That's a gently-massaged 12 year old desktop computer with a hight-end desktop-class CPU (for the day) and mid-tier GPU configuration by today's standards. You wouldn't want it as a new machine for high-end gaming or anything like that, but even 12 years in it's faster than the new M1 iMac (which is also a *great* argument in favor of upgradability): The new M1 iMac Mid-Tier (so dual cooling fans) nets a Cinebench R23 CPU Multi Core of 7722 and a Geekbench Metal Score of 21525. Don't get me wrong, that's great performance for a low-power CPU with integrated GPU, but it's still slower than a mildly-upgraded 12-year old desktop. Only in single-core CPU tasks is it faster (not really a worry in *most* of today's computing tasks, but WOW those M chips sure are great at single-threaded workloads).
Now look at something like my VR gamerig which I'm currently rebuilding after a typical PC 4-year motherboard lifecycle failure (and that wasn't a budget board or anything, a mid-tier gaming board! Why *can't* PC makers build for longevity at any reasonable price-range?) AMD 5900X and (luckily I already own one) 2080 Super. The Cinebench CPU Multi Core Score for the 5900X is 22046. I can't give you a Metal score for the 2080, but in gaming benchmarks it typically scores better than double the frame-rates of the RX 580, but let's just call it 2.1X, which would equate to a Metal score of right around 100,000. And that CPU is not close to the fastest possible for professional workloads (I believe that is currently still the AMD Threadripper 3990X with a Multi Core Score of 75,671!!!), it's just the best bang for the buck in terms of gaming CPUs right now. The GPU is also an entire gen old at this point (but effectively impossible to upgraded, even if I wished to do so, due to zero availability of new GPUs).
So before Apple can build true PRO level Desktop Class machines they need to see massive performance improvements in their M chips. Until then I think they will almost have to stick to Intel (or AMD, not that this is likely to happen) and discrete graphics for their pro desktop lineups. In the end that's good news for all Mac owners using Intel, as the longer Apple continues to build even a single Intel based model the longer the OS and App developers will also continue to support them.
