Personally, I have the exact opposite opinion on optical viewfinders. Optical is eyestraining, non-ergonomic, physically limiting (can't use an optical viewfinder when you're shooting at foot or waist level without contorting yourself into weird positions), and modern shooting modes often require a lot more setup and selection than is comfortable to use without a full-size screen and looking at the controls you're hitting.

That last point depends strongly on the camera. I almost never have to take my D500 away from my face when I'm setting up a shot, because it has physical controls for the exposure parameters and displays in the viewfinder to show changes of setting - not just shutter speed, aperture, and ISO, but even down to stuff like bracketing and flash exposure compensation. Granted, a lot of DSLRs, especially the prosumer ones, don't benefit from such thoughtful (and expensive!) design. But you absolutely can get cameras that are ergonomic to operate.

(Well, if you buy Nikon, anyway. I don't know who does Canon's UI design, but whoever they are, they could be a lot better at it...)

You can buy a 90 degree viewfinder adapter if you really want to shoot low down. They're not very expensive.

Optical viewfinders also vary quite a bit in size which impact eye strain quite a bit - unsurprisingly the larger, brighter ones are typically in the more expensive cameras. If you ever get the chance, try a top of the range DSLR viewfinder next to a bottom of the range one. You might be surprised at the difference.

You can't use an EVF at foot or waist level either. That's not what I am on about. I have no issue with using the large back display, articulating or not, as a live display when needed. But when I want to use a viewfinder I want it to be optics, not a tiny screen in a tube simulating that view. And like @throwanem said having tactile buttons/dials available as no look controls is a thing a lot of people like to use over touch screens. I understand it's a personal preference and an increasingly minority opinion.

Now's a pretty good time to be looking at your local camera store's stock of used bodies, because mirrorless is already displacing them among pros and hobbyists who prioritize gear acquisition over technique^W^W^W^W^Whave already made the switch. It'll be a year or two, I expect, before we see D850s and equivalently high-end Canon bodies start hitting the used market in numbers, but aside from that, things are already pretty flush.

(That said, check the shutter count, and the rated shutter life of the model, before you buy a used body! That's the primary wearing item in any DSLR, and the cost of replacement means you're pretty much better off just replacing the whole body instead. You shouldn't expect to pay as much for a used body with a high shutter count as you do for a low one, and if someone tries to get you to, you're likely better off shopping somewhere else.)

You don't have to use the tiny screen. Mirrorless cameras generally come with a large display. I almost always use the display on my Olympus because it's easier and I don't look like I have a machine on my face (this was also an important advantage of old view cameras with the focus screen on top, like the Hasselblad 500). If you want, you can attach a much larger screen to your mirrorless camera, they just use HDMI output so get as big a display as you want.

I meant specifically when using a viewfinder on a mirrorless camera, it means looking at a tiny LCD/OLED screen inside an eyepiece that simulates the classic view vs through glass lenses/prisms for an actual optical view. I like optical viewfinders not little screens in tubes.

Sure but just because it's there doesn't mean you have to use it. It's there for nostalgia reasons, mostly.

I disagree its nostalgia. I prefer composing that way and don't use it "because it's there", rather because I like to. It's more comfortable for me. It's also not bothered by sunlight washing out the display. It's not some impractical relic used for old times sake.