If you have listened to or read the actual speech, beyond just a few snapshots taken out by newsagents, then you realise that what Rishi is proposing is not unreasonable. No one is talking about taking support away from people without anything else in place.
Rishi is talking about putting support in place for people to assess what work they can do and to shoft the focus away from what people cannot do. He is also pointing out, quite rightly (I work with young people and see this trend in over-diagnosing of normal feelings into clinical mental health illnesses - it's a self-fulfilling prophesy in many cases) that this is mainly about young people who are increasingly going from education into unemployment due to mental ill health.
Yes, he is also talking about a crackdown on benefit fraud, but that is not the main topic of his speech, just what the media latched onto.
Like I said, the man is trying to run before he can walk. We do not currently have the infrastructure in place as too many people struggle to get the mental health support they need, especially early on when early intervention could work preventatively rather than reactively.
We need a culture shift, though, and I have to agree with him on that. Because saying "they cannot do" IS infantilising when not coupled with "but they could do... instead". I know a fair few people caught up in the benefits system for a long time, who are convinced they cannot work because reasons when in reality, some are physically not fit (but could work from home), some struggle mentally (but could do routine factory or farming work) and some are plain anxious and just need help to push past their initial fears.
It's the support structure, though, that needs sorting first.