I agree with you that the idea MCID is scale dependent is ludicrous. What is the minimum that would be clinically important depends on what you have to do to get it. You sometimes see an MCID for a given scale as e.g. 3.1 points. This suggests, absurdly, that a simple, safe, inexpensive intervention that led to an improvement of 3 points would be “clinically irrelevant” whereas an invasive, toxic and expensive intervention that led to an improvement of 3.2 point would be worth using.