No official credentials but I am (late diagnosed) AuDHD, and have DC diagnosed in their teens with AuDHD/ADHD as well as a partner with (late diagnosed) ADHD. All private diagnoses (because I'm not in the UK and that's the only option).
First, "autism is a superpower" drives me completely and absolutely nuts. I haven't met anyone diagnosed with autism who considers it a superpower. These people must exist I'm sure but they're not people I now.
Second, I just don't believe you can buy an autism diagnosis unless we're talking about medical professionals who allow themselves to be bought (i.e. malpractice) which is a different thing entirely. I also haven't yet met an adult who is reasonably certain they're autistic who hasn't had that confirmed on a diagnosis (they must exist but I think rare). I think when you're talking about yourself you do know. It took me 10 years to go for diagnosis and the push was being in autistic burnout following the end of covid working from home (i.e. similar to the impact of return to school post covid on ND children a previous poster mentioned). Parents can be wrong though (they're working on observed behavior and don't actually know how their child thinks/feels), and I suspect it's fairly common that a parent thinks their child has autism, but actually they don't. I say this because I myself was that parent! I thought both my children were autistic, but I was 90% certain on DS, and on the fence on DD. DS isn't autistic: ultimately he doesn't have social communication issues (diagnosis was primarily 'he's a teenage boy' :)). DD is autistic. Ultimately, as someone who is autistic myself, I don't know what not-autistic really looks like. Stands to reason that someone non-autistic doesn't know what autistic looks like (absent professional training). Interestingly, my DS himself was strongly resistant to the idea he was autistic - i.e. he knows himself better than I know him (shock horror). Also interestingly, the psychologist said that they had never seen a better set of teacher feedback than they got for DD i.e. school not just missed it but completely and utterly missed it.
Third, personally, I do think that an ADHD diagnosis can be manipulated. There's not the same cross-functional approach to assessment as for autism. I can't comment for sure on the UK private assessment protocols, but my son's first ADHD diagnosis was made by simply asking him the questions on the questionnaire. Parents weren't asked to complete and there was no assessment by an independent party (eg school). Given that both children and parents don't know what 'normal' really is for an age, I do believe that I could easily have gotten the first psychiatrist to diagnose even if DS was NT (he was diagnosed again through an more intensive process when we assessed for autism). As I said, I don't know how it works in the UK, but I do personally believe there is over diagnosis of ADHD. I'm not even convinced I have it myself rather than having sub-consciously known what to say on the questions.
Fourth, bullshit an autism diagnosis doesn't help anyone. It's not magic but understanding yourself and knowing that your reactions aren't just because you're a bad person who is failing at life is massively valuable. The big things for me were stopping the obsession about whether I was or not, and helping me to recognize that I needed to allow myself leniency when recovering from a meltdown or similar. It's brought it's own problems in some ways as I'm working on accepting that I may have actual limits in reality that I can't force myself through by just working harder, but overall it has definitely helped. Obviously being autistic isn't an excuse for being a horrible person.
Fifth, I don't think I'd say autism is being diagnosed instead of mental health disorders. If anything it seems more common for adults with a long history of (diagnosed) mental health issues to be late diagnosed with autism, which often is a triggering factor for anxiety and depression in particular.
And finally, to go back to the original question - I think that the potential over-diagnosis of ADHD/uptick in people self-diagnosing as ADHD or other ND (or the trend that will hopefully die a death that you can somehow identify as ND) is fueling the fire that all ND is over diagnosed and that includes autism. People who have talked to friends/family about ADHD assessments don't realize how much more involved an autism assessment is. Even on this thread we've had people think that their experience of an ADHD assessment is somehow relevant to an autism assessment. Parents holding out that their kids are autistic without any form of assessment (and I do understand the waitlist issues) is also problematic because some of these kids have other issues or it actually is (shock horror) a parenting issue, particularly with over reliance on screens for children under 2. Again we've seen on this thread people refusing to believe that it is actually common for parents to be wrong. I hate the fact that all ND tends to get lumped into one bundle as if it's somehow all the same. And then we get back to the stupid 'superpower' type language! And don't get me started on 'neurospicy'...