High-end cameras have removable batteries. Instead of using a power bank, you'd normally pop the battery out and swap in a charged one.

This is assuming that you're taking pictures for a living.

Well, yes. Basically all mirrorless cameras have removable batteries. And changing them is obviously the direct way of keeping your camera operating continuously. But the one situation in which I can assume you would notice the difference in battery consumption is, if you are waiting for hours in a single spot to shoot rare wild life. This is where you might be watching for hours without taking any shots. But in these situation, you could add external power supply to not even have to exchange batteries.

Yeah, external power is the play. On current-gen cameras, you can run with USB-PD. I have a 256000mAh Anker bank and you can shoot 4K60 for a very, very long time on a Panasonic GH6. M43 isn't the best wildlife body, but I have to assume you can get some pretty staggering runtimes in idle on something full-frame.

Why would you say that µFT isn't the best wildlife body? There the long range lenses shine (do I need to say 150-400/4.5?). Also wildlife often means hiking through the wild, where µFT also has big advantages.

I mean, I shoot only M43, so I'd do it because I wouldn't have anything else on hand, but the smaller sensor does present quality challenges a full-frame doesn't and modern full-frames can get pretty small while still being able to pack a pretty long lens (though as you say, the long M43 lenses are pretty tiny!). If money's no object, I would think most folks would pick a Nikon Z9 or the like over an M43 option. You can totally get fine pictures out of the M43 in a wildlife/outdoors setting for sure.

(The smaller M43 lenses are a big part of why I use it, though, as I am a video-first shooter and Panasonic's video-friendly, silent-motor lenses are pretty ridiculous.)

Smaller sensor sizes, generally, work better with high zooms (with the exception of astrophotography). Look at broadcast TV cameras as an extreme example; those use sensor sizes even smaller than M4/3. It simply becomes impractical to build a very long focal length lens for a large size sensor that isn't telescope-sized.

And even when people are shooting with full-frame lenses (e.g. on Sony), they often use an APS-C camera body because it has more pixels in the very middle of the captured image, with the assumption being that you're going to be doing a lot of digital cropping anyway to get more effective zoom.

Huh. TIL. I am very much a video-not-film shooter and photography is a weird art I do not understand, so thank you for setting me straight!

I am also a µFT-shooter, e.g. OM-1 with the 300/4. I named the 150-400, a lens which certainly doesn't have a 35mm counterpart. At which point the "quality" advantage of a 35mm sensor becomes moot. And of course also, if one doesn't have a lens to match my more affordable 300/4. Assuming the 35mm shooter is even willing to carry the equipment :p. The camera body size of course does no longer make a difference, but the lens size still does.

Long exposure and video are an issue. One Nikon solved with second gen mirroless bodies which can be charged while on through usb-c and second gen battery handles that can do the same. Other than that, battery swaps are fast enough.

Considering with only one mirrorless camera and one DSLR I have to lug around 10+ batteries in my camera bag...yeah. Running out of batteries at a wedding isn't an option.

Plus I keep backups just in case my camera bag is stolen, and first party Nikon batteries are $70 per. I hadn't considered keeping a battery bank and a long USB-C cable as a backup though, so I'll need to look into that.

If you want to use an optical viewfinder, then yes it's relevant. If you spend more time framing than shooting, it could be very relevant.

I was referring to the battery life. If you want to use an optical viewfinder, that is a different argument. The question just is: how often is it really making a significant difference in battery life? One which you cannot reasonably compensate for with more batteries or external power supply.