The downside is that when your peripherals' batteries go flat, or the Bluetooth stack goes tits-up, you've got no keyboard or mouse.

This happens with some regularity on the iMac I'm typing this on presently. The system has a wired (non-issue) keyboard, but the wireless Bluetooth "Magic Mouse". Every so often the mouse (or more likely, Mac) decides it wants to wedge Bluetooth and forget the mouse pairing / configuration. Powering down the mouse doesn't fix this, nor does restarting the computer itself.

If it wasn't for the old legacy wired mouse+keyboard combo from the prior iMac this largely replaces, the system would be dead in the water when this happened.

(Subsequent OS updates seem to have made mouseless operation slightly more viable, at least for recovery purposes. It's possible to log in and fire up System Preferences, which is most of the recovery process.)

Too: removing additional steps in the keyboard/mouse stack results in crisper response:

https://danluu.com/keyboard-latency/

Fewer moving parts => more reliable operation.

I hate wireless mice and keyboards.

The downside is requiring charging or batteries. I just have a hard time justifying the hassle when I have no need to make those devices wireless.

And my battery always (when I had one) died mid-day, so it forced a work break, which isn't always bad anyway

These devices use so little charge that they basically need no charging. You can literally do it once a month for a couple of minutes.

The justification is not having any messy cables on my desk, where space is a premium and mess distracts me.

The problem is you forget every time so once a month you are in the middle of doing something and you have to stop, turn your mouse upside down and then wait.

You get a "low battery" warning that pops up when it's got a day or two of charge left so it's fine.

Don't need to stop anything, just plug it in at your next lunch break or when you leave for the day.

It's just not really an issue as far as I can tell.

The only way you can run into that is if you have a three-day meth burner (so, no breaks where you can charge the mouse), ignore the three days of low battery warnings, and then use it right up until failure yet be outraged that you don't have 5min to spare (meth will do that).