Soap Screen use and mobile radios are the biggest power hogs on my device (pixel 8). The idea that I'm going to leave the display powered while not touching it at all is just stupid. I noticed that my phone became warm after just a few minutes of desktop mode. In my mind desktop mode should be more efficient than normal use, but only if the phone is able to stay dark and power off the digitizer.
Google's roll-out of desktop mode has been pretty slow so far. It seems plausible that they will add features over time, so maybe they will do something along these lines.
Meanwhile, I am skeptical that the digitizer is a major power draw. Manually dimming the phone display and not recomputing its contents may help some, and blanking it would presumably save more... but... running the GPU to compute pixels for a large external display is going to be costly, and sending those pixels over USB-C at however many frames per second will cost power as well (thus generate heat).
Soap Maybe it's a stupid threat model, but an unlocked phone on a table is easier to grab and pocket vs a laptop. I'd feel more comfortable if my device was secured while docked.
I have no idea what Google's plans are for new features, but I am skeptical that they would soon add support for the device being simultaneously unlocked (external keyboard, mouse, and display) and locked (digitizer and internal display). I think it would be more likely that they might add an option that would lock the device if an external display is unplugged -- though many people would not want to enable that option.