> but the second citation in your first comment suggests that it's just on the research side that they should abandon current classifications, but that the DSM definitions are still needed on the clinical side.

Her book is largely focused on research, it has little to say about clinical practice. The paper she coauthored which I cited does argue, yes, that the current DSM diagnosis should be retained in clinical practice for the time being, even while they think it should be largely abandoned in research. However, I think their justifications for its continued clinical use are somewhat undermined by their arguments against its research use. That paper says "The ASD diagnosis remains necessary in the clinic to assign a child to early behavioral intervention and to explain a child’s condition". The paper argues that ASD lacks biological validity and construct validity – how can something which lacks validity actually explain anything? A diagnosis lacking validity must in itself be causally inert; and if the diagnosis is reduced to a symptom set as Waterhouse's book argues, then using it to explain symptoms becomes circular ("the symptoms are explained by the symptoms").

> Does she propose a better classification system, something like what she'd replace it with in the next DSM if it were up to her?

Her book proposes the definition of two phenotypes (not disorders) on p. 434-435. The first is "Neurodevelopmental Social Impairment Only Phenotype", which is basically equivalent to DSM-5 ASD criterion A (social impairment present from childhood), but with an exclusion for atypical sensory behaviours, atypical motor behaviours, rigidity in behaviours or interests, atypical language development (functional language impairment), ADHD symptoms, intellectual disability, developmental delay, or seizures. This phenotype is broadly similar to DSM-5 social (pragmatic) communication disorder (SCD), except it makes no claim to be a disorder, only a phenotype (symptom set), and also that it has broader exclusions – DSM-5 SCD doesn't exclude ADHD symptoms or seizures, for example, her first phenotype does. Her second phenotype is "Neurodevelopmental Social Impairment Multi-symptom Phenotype" which requires neurodevelopment social impairment combined with one or more of the exclusions from her first phenotype.

As well as criticising the DSM-5 for proposing a disorder as opposed to a mere symptom set / phenotype, she also criticises its two-domain model, which requires both social communication symptoms and RRBI/sensory symptoms for a diagnosis. She instead proposes a model with one core domain (social communication) and a set of optional domains, none of which are required, and of which RRBI symptoms and sensory symptoms are just two additional symptom domains out of several. She justifies this on the grounds of clinical experience that social communication impairment can occur without RRBIs or sensory issues – this clinical experience is reflected in the DSM-5 diagnosis of SCD.

The relationship between SCD and ASD is arguably one of the most poorly thought out aspects of the DSM-5, and in practice many people who strictly speaking should get an SCD diagnosis get given an ASD diagnosis instead, despite not actually fully meeting the ASD criteria – many clinicians are very hesitant to use the SCD diagnosis even when the letter of the DSM-5 would indicate it, due to SCD's lack of public awareness and unclear eligibility for funding. SCD itself is a strange lacuna in Waterhouse's book – despite SCD's obvious relevance to her argument, she hardly ever mentions it by name.