i wouldn't say it's at anyone's expense tbh
the point is that when you're playing marth, it's good to have movement fluidity but it's generally poor to express it with outward attacks because that kind of practice encourages the player to swing blind from neutral. the sample video from the OP is a good example of how "practice" can actively encourage players to develop these bad habits. and i can tell you right now, any time you NEED to L cancel dair, you're almost always doing something wrong.
@ the OP: there's nothing wrong with using solo practice to improve your movement and fluidity because that type of practice will aid your positioning and placement- the two things that make this character viable at all. I suggest that you only practice IASA on marth's dtilt because movement to movement timings are invaluable and you generally won't be using other swings from neutral other than dtilt. even if you're using marth's sword as an interrupt, it's not going to be with shuffle nair or dair. if you want to practice interrupts, practice them to have optimal coverage for movement after your swing so you can do movement > movement to negate the risk from an open swing. for example, if you must practice SH fairs, practice either fair speed to come out as fast as possible after the SH, do ledge jump fair as fast as you possibly can after the edge grab, or do fair > l cancel > dash away and try to minimize the time between those actions. those will make you a much better player while mitigating the formation of bad habits.