From what I see the problem is that the newer Macbooks are slimmer and have less efficient thermals. So the processors are more efficient, but the benefit gets lost by shipping them in smaller enclosures and on top of that smaller batteries.

I'd love to see the new processors and RAM in a 2015 enclosure. Makes me wonder whether someone has attempted to hack it together.

The dGPU is much hotter due to a no-downclocking driver bug that is worse on a chip that is more powerful, and bad software that wastes CPU cycles had more cycles to waste.

Newer processors do the same amount of work faster using multiple cores, so they burn same amount of energy in shorter period of time.

But newer processors use less power at idle, and doing work faster means it can spend more time at that idle, low-power state.

Yes, of course. Processors are idle most of the time anyway, but single processor has more time to dissipate heat passively doing same amount of work (e.g. 2% work, 98% idle), while multiple processors, e.g. 4, will use same amount of energy 4x faster (0.5% work, 99.5% idle), so fan will kick in.