I've killed dozens of mice and in every single one, left-clicking was the first to break.

No, that's not something that I've ever noticed. It never happened with any mouse I had before (even though I had a few mice fail mechanically in other ways in the past) and 17 years ago I bought two (one for home, one for the office) of the classic HP 3-button optical USB mice, which I've used more or less daily ever since without anything ever failing:

I find it somewhat hard to imagine that these mice could ever fail in any way, and even harder to imagine why the middle button would in any way be different from the two outer buttons.

100% of my failed Logitech mouses were left clicks.

I seem to always kill m1 and get a new mouse every 5 years

For me, yes

Try to open the mouse and clean it sometimes dust/lint gets trapped and causes this.

Middle click? Jokes on you I use a trackball.

The most common mouse problem I experience is when the left click or right click buttons intermittently double click on a single click. The second most common problem is when a button won't stay "down" when I'm holding down the button, causing erratic drag-and-drop behaviour. I've actually never experienced your middle click problem.

Didn't really use it that often, and it stopped working right as I was playing Sekiro, which was annoying because I had to remap the target button elsewhere

If I had to take a guess, it's probably the most complex mechanical part in a mouse. The other buttons don't have the added extra movements. I've had the same Razor Deathadder for like 15 years, still going strong. I'm just glad they don't have actual rolling balls in them anymore.

Yup, for me it is the middle click the reason I'm buying new mice.
Tho I don't think many people are actually aware of the utility of the roller click. This is why I usually pass the broken ones down to my family lol

So far I had no mice break on me. And my most used buttons on the mouse are probably left, middle and then right clicking (I like opening new tabs instead of going back and forth).

Maybe too much pressure you put on your buttons?
Edit: I had 5 mice (at home)
Roccat Kova
Roccat Kone XTD (I think)
Logitech Mx Master 2s, 3 and 3s.

And I replaced the MX Master mice only because the rubber coating got sticky and I RMA'd it with Logitech support.

Yeah, though this time my g502 is starting to fail with mouse wheel tilt click

Mine failed that way, yeah. Though I think it was user error after trying to repair a mouse that wasn't designed to be serviced officially. All the rubber wore off the wing, so I had to disassemble and replace it with a generic part. The left and right clicks are a bit spongy now, and middle click only intermittently works now, but the mouse is grippy again. I wish more mice were actually repairable.

It's annoying to have mice that break down and need to be repurchased instead of a few replacement parts. 90% of the mouse was fine.

It's a shame they don't use the same tech used in the cell phone screens. Not putting a screen, only the sensor. My cell phone is working good after years of use.

I've had good luck with rubbing alcohol to remove the gooey mess. You'll end up with bare plastic instead of the rubber grip, but that's an improvement over sticky.

I replaced my last mouse because the mmb was getting excessively squeaky and wouldn't scroll unless depressed, which made map navigation/zooming in certain games difficult.

I use the middle click extensively. Probably as much as left click. I've never had it fail. Usually one of the side buttons goes first

Nope. Right click every time.

Finally bought a mouse with optical switches.

I use middle click navigating my browser, but I also rebind Reload to MMB in every shooter game I can. I’ve been through more mice than I can remember since I started rebinding it but most of them were actually replaced for other reasons. I’ve only had 2 with faulty middle-clicks since probably 2020

On my current mouse it's the scrollwheel that's slowly failing but middle click works fine. There's also a box of non-functional mice in my house. Last time I checked it contained one mouse with broken middle click, one with broken left click and one with broken right click. So I guess I disagree with your theory.

Attempted fixing my wheel click a bunch of times by cleaning and lubricating the microswitch, it would improve for a while and go back to how it was. Missed clicks, multiple clicks, etc.

My mouse has a bunch of smaller buttons all over it, I ended up binding wheel click to a smaller button near the right button until I found a microswitch that suited, but I'm just really comfortable with the way I have it now.

Also, depending on the type of rubber coating, cleaning with isopropyl alcohol may help with the stickiness.

I find the middle click button awkward to use so I typically rebind the forward side button to middle click

I use a touch pad, so I don't have this problem