I would argue it's rude to promote 7 knights. Just checkmate and move on. You shouldn't make someone sit around.
The player with the 7 knights isn't forcing their opponent to do anything except make the choice to resign or play on in an obviously lost position.
If a player is going to have a "never resign" mentality, they need to realize that they risk moments, where their opponent isn't seeking the quickest victory, and stop acting like a self-righteous victim when it happens. The resign button exists for a reason. Save some time, take the L, and move on to the next game.
I think it is okay