Both of us are doing some guesstimate here without detailed justification. Not sure it's worth the digression. Perhaps you're expecting a justification from mine. So here we go.
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For your 8500B/8700B, the sustained max power is about 65W. From benchmarks I've seen, M1 Pro in gaming can sustain ~60W. Gaming hardly pushes both CPU+GPU to their limits at the same time. ..... Say it's 65W. I don't know if you're happy with me hypothetically put it there for M1 Pro.
Apple isn't known for provisioning adequate cooling for Intel Mac (except MacPro7,1). When 8500B/8700B is sustained at 65W, temperature is around 100C. Fan is spinning up. Noise and throttling crank up. Not something Apple wants for their new Mac's.
So the current 2018 era chassis is capable of cooling 65W. This justification is essentially agreeing that that it
does indeed fit. Your baseline justification is has little directly to do with thermals and more so do with noise.
Three super major problems with that as it being an Apple "non starter" criteria. First, is that Apple shipped the 2018 Mini in the first place. The noise can't be sorry horrible that users in large numbers are going to reject the product. It worked before , so therefore is clearly could still work now.
Second, among "average Joe" users who in the world operates their Mini at 100% sustained power draw for 5-6 hours blocks of time per day. About no one. The bigger deal thermal load that the 2018 cooling system had to cover is the P1 , P2 power spikes that the 8500B-8700B would do that push way past 65W. Intel's default TDP numbers are about "sustained" which implicitly is where the clocks are dropped to base clock levels. You can get fan ramp just by doing burst loads just for several minutes at P1 power, which is higher than 65W. The other "accounting" problem is that the T2 was a separate chip consuming TDP before and that gets subsumed into the M1 Pro chip. M1 Pro also has more cores ( 8 cores ( 10 if weave in the E cores) versus 6). If you ramp the M1 Pro 6 cores it isn't going to be at 65W ( again in the "average Joe " context where they have some 4-5 core constrained app it is just going to run cooler. ) . So "average Joe" who overbuys into a Mx Pro Mini.
Third, it really isn't "average Joe" this product is primarily aimed at. If 40-55% of the buyers are folks provisioning these systems into data center rack deployments , departmental servers , etc. then they are not going to be placed on people's desks ( and near their ears). "average desktop PC" noise is not a data center deployment show stopper. If a Mini is located 10 ft or 100 miles away from the user then the noise isn't going to make a material difference to the service it provides. The Mini form factor is more efficient with a Mx Pro class SoCs gets more "CPU cores per square foot of rackspace" than the Studio can. The Studio is 3x bigger and only a bit less than 2x increase in the number of cores when using a Max ( and the Ultra is more 2x bump in $/mac. ).
"Bu there are 45-60% of the users that are not data center deployment". Yeah but that noise threshold is also an Apple mark not necessarily an end user mark. The general PC market has lots of folks who have systems as loud as a Mini 2018. If do some simple things like attach the Mini to bottom desk surface ( incrementally farther away from user's ears with some sound deflects. ), then it is even more in scope. There are decent number of folks who will say "Yeah it would be "nice" to be quieter, but I have work to do". Folks who can chuck the eGPU because the 16-20 GPU cores is not enough will likely be happy to make the trade off ( two fan noises down to just one and more desk space at lower cost. Those folks will likely buy. )
For the user who put a super price premium on noise they would have other options on either side of the price point. Plain "Mx" Mini (lower price) and Studio (with Max "floor" starting point with a higher price). If noise was a show stopper for them then they could move to a different product in the Mac product range. That is just product segmentation.
The notion that Apple removed "X from product Y" so they'll have to remove it from all of the Macs is flawed. No Ethernet jacks on laptops has not nuked them from most Mac desktops ( iMac 24" has a way to easily put it back). No headphone jack on iPhone has not lead to wide spread removes from Macs. Apple has not removed stuff where the "thinness politburo" hasn't chopped so hard as they had to make space trade-offs. If use the old space in the current Mac Mini chassis there is no "thinness" chop problem. The space and vents are there. Apple doesn't have to remove noise here if that brings in $100M in profits in server room deployments.
I would think M1 Pro is indeed likely hitting the thermal design limit if put inside the current Mac Mini.
Which is not really a good reason to kill the product. The chassis/cooling was designed for that limit.