Background: I bought an 80D when it first came out as a dedicated camera for my 100-400 II. I already own a 5DsR for everything else. The copy I got had a lock-up problem so I sent it back. I was going to exchange it when I became aware of the future release of the M5. So, I returned the 80D and as so many have done here, waited for the M5. The logic of switching to the M5 being that it would be smaller than the 80D and would fit along with my other stuff into my crowded backpack mounted to the 100-400. The M5 seemed to have many of the same features of the 80D.
So, checking prices and so forth this morning, I checked on camerasize.com for the size/weight comparison. Actually, with the adapter, the M5 combo will take up slightly more backpack real estate than the 80D combo length-wise (the most important dimension in my backpack environment) according to camerasize.com assuming the pics are accurate. The M5 will still probably weigh less than the 80D but I did not see any weight specs of the M5 with the adapter mounted.
Also, depending on what price you find the adapter for, $199 or $99, you could be paying more for an M5 than the 80D.
The M5 checks most of my boxes with the exception of an annoyingly limited AEB, the native lenses and this new-found information. I owned the M3 for a short while along with the 11-22 & 22mm. These two lenses left me wanting (no offense to anyone who loves these lenses). So, I don't think there are any native lenses to draw me in. I was enamored of the 100L II on the M3 though.
I'd love to see some L-type lenses released for the EOS M mount. Yes I know, people have longed for L-type EF-S lenses for a decade and a half and have gotten squat.

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Once you've done fifty, everything else is iffy.