This is another aspect to this that is not being talked about.
We knew immediately that Rashford was obviously offside by virtue of watching on TV but Akanji, Walker, and Ederson didn’t know for sure. Which means everything they did whilst play continued had to be based on the assumption he was onside, because if he was, and the went in for a challenge based on the way Rashford was shielding the ball, they could give away a penalty and/or be sent off.
Only people that have never actually played football will think Rashford isn’t influencing (obstructing) the play of the defenders and keeper in that situation.
All of the uncertainty and risk was with the defenders in that scenario, meaning allowing that sort of offside play further disadvantages the defending team (who is already inherently disadvantaged).