When i did my GCSEs , it was about 25% for a C and that was an "easy" paper iirc. If a C is in between a 4 and a 5, then it doesn't seem like there's much difference.
I think more people need to fail and not out of malice, but i think it's a good way to show pupils that they need to work hard and not only in test or school settings (my grades were disappointing and it made me work harder). People often put in little work because "it's easy to get (whatever passing grade) anyway", and if the boundaries go up and the failure rate increases, it stops people from thinking this way and letting that sort of thinking be their norm in life. Now there are people who work really hard to get a 4/5 grade who this would negatively affect and they wouldnt need to do better than this; because they don't need a higher level of the subject and already worked hard; so the solution would be a difficult one.