Rule was to keep offenses from going too fast when spread offenses were in full swing (I.e. Oregon or “go fast, go hard). Offenses were subbing quickly, snapping the ball and catching defenses off guard, so they made a rule change to allow defenses a chance to sub.

In this case, if offenses used a delay substitution, it wouldn’t offset the purpose of the rule because the rule was in place to slow offenses down. But it would be a different variation of abusing the rule