I genuinely believe that something needs to be done, given the rise in claimants - but that's because no decent society should accept a situation where, for example, there are 5 times as many people who can't work in some parts of the country compared with others. That's society failing those people, and arguing that we should just stick with the status quo is cruel.
However, I don't think you're right to compare the three people you're discussing, to a £75k earner. That makes it sound like they're rich.
If it's £4400 total, then that's about £17.6k each, which is equivalent to them earning £19.5k a year before tax. That's quite a lot less than minimum wage for a 37.5 hour a week job (which is over £22k).
So, you've got three people, with disabilities, who each have less money that someone on a full time minimum wage job. Knowing nothing about their disabilities, I would say that seems reasonable, rather than unsustainable.