It is unfair, that is unarguable. It should be obvious but I guess I'm the only one that sees the obvious that one side having more players breaks the balance of the game.It's an advantage, but it's not unfair. People get injured, teams deal with it.
The only reason it exists in its state is no one has thought of a system that isn't open to abuse to replace injured players.
Beating England 11 v 10 is an entirely different scenario to beating England 11 v 11.
Bangladesh beat England with one more player, so they have only proved they can beat England with that advantage, they haven't proved they can beat England in an equal contest. Its that simple.