For those suggesting that PIP is a tightening up of the system due to fraud, it isn't.
The philosophy behind PIP is in line with the Tory value of 'pull yourself up by your bootstraps' ie that everyone can improve their lot if they work hard enough. The belief is that the right thing to do is give people a hand up, BUT that people should then be able to do it by themselves. I'm not being facetious with that - I researched universal credit and PIP prior to its introduction, and that it one of the fundamental principles of the Tory benefit system.
This philosophy underpins the Tory welfare system. It is the reason for reassessments, as the Tory view is that 'anyone can adapt'. The example in their literature was about a war veteran 'adapting' to life with a physical disability and the assumption that once you've got certain aids and adaptations that you will be more independent.
This reasoning is also behind why a lot of welfare services - parenting support, homelessness, mental health support worker, supported housing etc - are very time limited (eg 12wk, 6month or 2yrs). The expectation is that you train people up and they should be fine. The Tory philosophy is that to do otherwise condemns people to the scrapheap and creates independence, and that this system respects the individuals ability to change, and their self determination.
The way they wrote it sounds almost ok, until you remember that there are huge numbers of people with severe and enduring illnesses/disabilities that don't improve AND that we as a society do not adapt to that individual. It leaves the onus entirely on the individual to change and does not recognise how inaccessible the world is, or what barriers there are (eg lack of employment or education, benefits trap due to cost of housing etc)
Reassessment doesn't save any money either. Any 'savings' from getting people moved over to cheaper benefits is swallowed up by the cost of monitoring and reassessing.
I'm sure it's obvious that I'm not a Tory supporter, but this concept of 'everyone can adapt/get better' is genuinely the reasoning that underpins these policies.