I generally read this thread for fun stories. I do feel obligated to remark though on those commenting that the laptop bag, backpack, purse etc should be kept under the seat or that passengers should feel any obligation to do so. Moderator, my apologies if this entire conversation and post are misplaced.
Perhaps those on this discussion board have forgotten that a regular economy ticket (not basic economy) on every major US airline now requires those without status to PAY for checked bags. Up until 10-15 years ago this was NOT the case. One could purchase ANY economy ticket and I believe check 2 bags no charge. At this time, I was a graduate student with no status on any airline. For both domestic and international, I often checked my bag so I did not have to carry it around the airport, not worry about fluid requirements, and happily board last (minimizing time on the plane). And when I did carry-on my rollaboard, while I may not have always got a spot above my seat, I never recall being forced to check a bag and never recalled so many people being forced to gate check.
When the change was made to start charging economy ticketed non-status passengers for the checked bag, guess what I did...I started carrying my bag on every single time. When I thought I needed to pack more, I packed tighter and made cuts so it fit on a rollaboard. And I can tell you that I distinctly remember seeing more people carry on and more issues with bin space.
Well for convenience purposes (given regular economy seats are already tights on leg space), I decided I would not let the changes the airlines made to make more money (100% their prerogative to seek profit maximization), force us to sit even more uncomfortably. A few times FAs in the beginning tried to have me move my backpack. I would politely point out that the overhead issue was all because of charges for checked bags and that passengers are being forced to suffer because of it and that placing my backup under my seat (31-32" pitch seat) was uncomfortable. The FAs often commiserated that the change made their lives harder and made boarding more of a hassle. I felt bad for the FAs. I soon after got airline credit cards for baggage check and priority boarding. Though to be honest, I now avoid checked bags regardless because of all the luggage issues
Long story short, I am hopeful people do not forget the origin of why the overhead space is such a challenge and think twice about making such strong statements that laptop bags, backpacks, and purses belong below the seat. Passengers 100% have the right to place those bags in the overhead space and it is not a DYKWIA (though OP did point out that OP shared the story for different reasons).