I used to suffer from agoraphobia, anxiety and depression and receive DLA so I understand the additional stress it causes people on a low fixed income when changes are mooted and folk discuss it with little regard for how it feels to be on that safety net that they are discussing moving/removing.
I did recover from those MH conditions.
I do think that money would be better spent not on the rejecting applications and appeals process or worrying about whether some disabled person somewhere might be getting a little bit too much (which would tend to act as a stimulus anyway as it's most likely spent in the local economy rather than stashed away in an offshore bank). Still keep the existing fraud folk who are able to investigate anyone who is gaming the system.
But the rest of the money I think needs to go into helping people recover from mental illness, with better treatment, community support and gentle (rather than pushy) help to support those getting better who are out of work into training, education or careers which build their competence and confidence and are suitable for any long lasting limitations they have. Not about compelling people who aren't ready or able, but supporting those who are (agency is so important for mental health) to do meaningful work.
An approach that genuinely helped people would not just ease human distress, it would also be financially worthwhile in terms of tax revenue and reduced benefits /NHS costs.
I think we need a humane system that supports people well and helps them recover. Not make their MH worse with great stress, mistrust and punitive threats to withdraw how they pay for the basics they need to live. But the answer is also not just giving folk payments for decades without actually helping them get well again where that is possible.