The needless processes / bloat still burn electricity though. I'd have to guess that given the millions of installed macOS machines it's a non-trivial amount of wasted electricity. Long gone are the days of ruthlessly optimizing software for the limited hardware.

Apple has done more than anyone to make its hardware more energy-efficient and its software too. It even warns you about which apps are using the most power.

macOS is far from perfect, but when the background services are working properly, I don't see any evidence that they're any significant driver of energy usage.

On the other hand, when they're buggy and suddenly start consuming 100% CPU all the time for no reason...

Indeed, these processes are not all sitting there doing nothing.

Two processes in particular have been this exact sort of problem for me: mds_stores and mediaanalysisd. On three separate Macs (all Apple Silicon), I've observed the case heating up whenever the computer is plugged in but not actively being used. Assuming Activity Monitor is more or less accurate, the culprit seems to be those two, who always have massive amounts of accumulated CPU time, but never seem to actually be using CPU when watched. I suspect, given what they supposedly do, that they're also needlessly exhausting SSD write cycles, but that's harder to analyze/prove. Naturally, they are also in the untouchable area of the file system. Completely disabling Spotlight, which you can do without disabling SIP, seems to always fix this problem, albeit at the cost of seriously decreased usability. I've also had mixed results with just limiting the categories of Spotlight indexing in System Settings.