For something that prescribes how to do everything, like Scrum does (e.g. you need all these meetings, this person does X, this person does Y, teams have to be cross-functional, no specialists, etc.) I think it's more than fair that the burden of proof lies with the proponents of that methodology.

Another way of putting it is that there are many good ways of doing project management, as many good ways as their are unique (team, circumstance) pairs.

Most people have an intuitive understanding of why some very low-overhead, low-formalism, "just get stuff done and be reasonable" workflow has worked really well in some specific (team, circumstance) combo situation.

If you want them to override their experiences with how those ad hoc, customized management processes worked well, and instead believe that a company-wide, one-size-fits-all approach that removes tons of workflow autonomy that could just be left up to separate teams to customize for themselves, then you absolutely have to have extremely convincing answers to address widespread bad experiences.

What if scrum proponents are busy having successful experiences, but are open to an argument convincing them that they are not?