That's a rare example around the margins, but fine IMO. If I'm in the front of a 737, I'll generally shuffle my roller bag back a bin and it works in rows 2-3 and leave the first bin for bags that are a bit undersized. 99% of the cases where people store bags sideways in bins designed for wheels-first are because their bag is oversized, but there are a few outliers due to the curvature of the plane.

I'd say it's closer to 50% oversized bags and 50% lazy/thoughtless passengers.

OP: If it was more than half full, your bag was way oversized, and I've have given you a look too, especially on the A319. Comparing it to somebody else's carry-on plus personal item just proves the point. The personal item is supposed to go under the seat in front of you, unless you're in the bulkhead -- and, even then, having two separate items makes it possible to arrange them in ways that aren't possible with a single, larger item.

Liters are a measure of volume, not weight or mass.

Most European airlines go by weight, but in my experience they completely ignore backpacks.

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.

Why is it relevant to call out the mention of "petite"... I agree with the rest of your points.

If your rollaboard is compliant, then it will fit wheels-first on a 737. It may not fit on a regional jet, but that's another story.

Not over Rows 1 and 2 on 737-700/800 and definitely not in Sky Interior first row bins. The 737-900s (non-Sky) have bin extensions that will allow it to fit somewhere over row 2, but the curvature of the fuselage limits the space in the first few rows. Every other fleet, no problem.

The bag fits in the newer sizers, though I realize there is a bit of a margin there.

And the personal item should go at your feet and the roll-aboard in the OHB. So it might be similar in total volume but the 75L backpack is taking excessive OHB space.

While premium cabin comes with enhanced checked luggage, it does not comes with enhance carry-on capabilities.

It is when you then claim this was justified "because I paid good money for my Z fare."

That's 43,120 cm^3. Or 43.1 L. A full or even somewhat full 75L pack is nowhere close to conforming with the size limit.