75 liter backpacks are HUGE. Granted, my main backpack when I was actively camping is a little bigger but that is WAY beyond a legitimate carry-on size (never mind the fact that the awkward shape means it takes even more room than 2 regular rollaboards would). I don't care how much you paid for your ticket -- unless you bought an extra ticket, you SHOULD feel guilty for taking that as a carry-on. I've seen people bring full-size backpacks on for decades and always wondered what went through their minds thinking those were legitimate carry-ons, how the gate agents missed it, and why the FAs didn't object.

Claiming a cursory visual inspection says it would be similar in profile to a medium wheeled carry-on with personal item attached is just nonsense and you should know that. In the first place, that personal item is separable for easier stowage and the handle retracts. In the second place, there is no way a 75 liter backpack comes even close to that profile. "Technically fit the dimensions"? Not even close.