Problem is still the 'hot spot' area and overall die area though, right? And with a slot, you push the memory and PCIe devices 'further away' electrically too, which I'd think would inhibit performance?
Die size is different, but not hugely different.
Ultra 9 285 is 243mm2
For comparison, nVidia 4080 is similar TDP (not exact) and die size of 379mm2
So yeah, the GPU is bigger, but not like a 4090 is bigger.
And that's before we get into the fact that not all that area is heat producing - that's just the total silicon die. But yes, larger area is generally going to mean easier to disappate the heat. That said, that's part of the reason these chips use spreaders.
Going to a slot - yeah, it makes some things more difficult. But it also allows you to mount huge slot-sized coolers like the GPUs have. That would be difficult to do if you keep the CPU horizontal and in plane with the rest of the motherboard. Not impossible, but you definitely have to move some stuff around to accommodate. But the more I think about that, even, that really only benefits air coolers that need that room for fins as close to the die as they can get, as heat pipes are only so efficient.
Move into water blocks, and that all becomes kinda moot, as water can move the heat to where ever you want pretty darned efficiently, so long as you can get the heat transfer to the water to work out. So I concede - you may have a good point about water blocks, which was your original statement.