I don’t think it’s needed or a good thing to have a one rule fits all to determine good from bad changes. Simple minds see the world simply and make simple rules to guide them through it. I think it’s logical to look at each change individually and make a determination. I don’t think most of those against dual spec have any heuristic. They’re against dual spec so they make up a “reason” then cobble together rationalizations for the changes that have already happened to fit that “reason.” Mostly people want to “win” the “debate” and that determines the arguments they use in any situation.