There's no doubt that Apple's made some MASSIVE strides with ARM over these last few years, but the fact is, if you were to stress out an ARM chip as much as you can stress out an equivalent Intel, it'd get just as hot. It's not like ARM is a vastly different architecture that's somehow magically 1000x more heat and energy efficient. If anything, they're both becoming far more similar than dissimilar these days.
There's only so much you can do with silicon before you start running against the laws of physics. Right now, we're about ~10 years away from 2nm chips, which is about the theoretical minimum size limit of a CPU. We're talking a scale so small, the distance between two transistors is about the thickness of 8 atoms. Once companies get to that point, any efficiency advantages will have more to do with the CPUs instruction set than it's architecture. And here, you won't see too much difference.
The key is TDP and cooling. It's a fact that SP3/SP4 cooling system is barely able to cooling those 15W chips without scorching hot on the back surface. On an iPad, ARM with 100% utilization does not produce scorching hot surface because it doesn't generate 15watts of heat and there's enough surface to dissipate all of the heat generated without having a huge temperature difference between the back surface and ambient temperature.