I get the need to make everything realistic to a fight. I think shadow boxing (other than sparring) is much better to emulate a fight with and to be critiqued as such. Think about it, 70%+ of your shots will hit air, so learning how to get used to that without losing your balance and maintaining your form, whilst also moving head and feet, is perfected by shadow boxing outside of sparring, more so than bag work. That feedback and extra balance you get hitting a bag is not as close to the real thing as y’all think, so stop treating it like it’s anything more than a training tool that can be used subjectively. If you like emulating a fight on the bag that is good too, but a lot of people use sparring and shadow boxing to emulate that and use the bag as strictly a conditioning/form tool (which is what I also do).
If there are glaring problems in fundamentals in beginners then sure, critique them for the basics like their guard or head movement or angles or whatever. They need to focus on everything. However sometimes I see people who are intermediate or advanced, using the bag as a training tool in a way that suits them and they usually specify the agenda of their work. Even so a lot of this sub just gives the same “cut angles, more head movement, keep your hands up, not enough this not enough that, pretend it’s a fight, etc” when the person is clearly just using the bag to work on something specific like conditioning or form, or power, or speed, or even a combination or movement.