If you're traveling domestically and don't want to check a bag, that's fine - but that doesn't mean that your bag takes priority over my backpack if it's in the overhead bin first.
Want space in the overhead? Find a way to get in an earlier boarding group. Don't want to have to worry about finding space in an overhead? Check the bag. If there's no room, it'll get gate checked anyways.
If I'm on sooner and I decide I want to put my backpack in the overhead bin (not usually the case, since I've got my personal entertainment devices and whatnot in there, but I definitely will on a short hop on a CRJ where I'm happy to stare out the window for an hour),
tough s____.
A is out of line.
A: I get delta rules may say first come first serve but I feel like it's also "don't be a dick" as a rule.
B: I’d argue that you’re violating that rule here
Preach, B!