There should be triage. Profound needs should be prioritised more highly. "Try this" self-help is a legitimate response for those waiting for assessment.
At the same time, in real life, we have people who fall in between A,D and B,C,E. We have people (F) who are still working but who are struggling with the move to a new office where the lights are bluer and brighter and there are fifty people in one big space when previously five people each shared smaller spaces, and can't access reasonable adjustments because, without a condition to declare, you can't request them[1]. We have highly-qualified people (G) who aren't working and who are struggling to get and keep jobs because they can't maintain the pretence of "normality" well enough and keep falling probation (a relative was like this for decades) and keep getting sanctioned by the Job Centre. We have new parents (H) who could cope before they had kids and now can't because kids are noisy and they aren't getting enough sleep and everyone they talk to is saying "you just manage, that's parenthood".
Chucking everyone bar A,D off the list ensures that the people I describe won't get the help that they need, It keeps G on the dole, puts F on the dole, and results in H's children ending up in care.
If we funded mental health properly (and I count neurodivergent services as MH) then we could assess F,G,H. Underfunding healthcare whilst funding nuclear submarines is a political choice.
[1]: "But reasonable adjustments are needs-based!" Yes, they are. And if you read the Equality Act, you'll find that an "impairment" has to exist in order for the person to be considered "disabled" to be entitled to the reasonable adjustments in the first place. That "impairment" is the diagnosed condition. I can't just say "I need you to fit opaque blinds because it hurts my eyes", I have to justify with a named condition why the light is a problem for me when everyone else seems to be fine.