Clinician and researcher in mental health here. In my view, you're being too charitable.
> The groupings persist because they are useful in facilitating communication regarding potential complications, prognosis, and patterns of response to treatments.
These are reasons, but they're a distant second. Mainly, they persist because those groupings - diagnoses, in other words - have become necessary for billing. The DSM, for instance, is an economic document, not a scientific one. For most people, it sits on a shelf, gathering dust.
As for "the best we've come up with so far", you'd perhaps be interested to learn how often the decisions about these groupings are made because of politics and economics, and not by science [1].
> The groupings persist because they are useful in facilitating communication regarding potential complications, prognosis, and patterns of response to treatments.
These are reasons, but they're a distant second. Mainly, they persist because those groupings - diagnoses, in other words - have become necessary for billing. The DSM, for instance, is an economic document, not a scientific one. For most people, it sits on a shelf, gathering dust.
As for "the best we've come up with so far", you'd perhaps be interested to learn how often the decisions about these groupings are made because of politics and economics, and not by science [1].
[1] https://www.amazon.com/They-Youre-Crazy-Paula-Caplan/dp/0201...