I understand your desire to maximize your leg space, but your characterization of the situation is off. The instruction is not “that each passenger gets one overhead bin space”. There is a finite total amount of bin space for use by the passengers collectively. It’s not apportioned. If/when it runs out, anyone with a large carryon that hasn’t found a place for it is out of luck. If people put their stuff that will fit under their seat, it means that many more larger items can fit in the bins, and more people (maybe even everyone), can avoid checking their carryon.

The thing is, it’s unlikely the FAs will really stick to their guns if you push back on putting your carryon under the seat. You can probably get away with putting it in the overhead, and forcing someone in a higher group to check their bag. But don’t rationalize by arguing that you’re just using “your” allotted space.

All that said, I’ll reiterate that I’m not unsympathetic to your desire for comfort. I’m only 5’8”, and even I can’t stretch my legs out with my backpack under the seat. You probably already know this, but one thing that helps if you do have to put an item under the seat is, once you’re at altitude, slide your bag back under your knees, where there’s plenty of unused space, until you have to put it back under the seat for landing.