Plenty of RSPCA "rescued" animals find new homes "off the books" so to speak, although obviously as an employee of the RSPCA you wouldn't want to get caught doing that as it's against their policies and procedures. And there are plenty of disgruntled ex-employees out there with tales to tell, relating to the management structure, employee welfare and training, animal welfare, policies, you name it.
They do have to put down huge numbers of animals because there are just so many unwanted animals out there, and there isn't the funds to house them all. A large majority alas are unwanted pets, or from people who can't afford treatment for their own animals. The latter often try to blag treatment costs from the RSPCA by claiming that their own animals are actually strays etc. Bringing back dog licensing would be a good start to help combat this.
The RSPCA as an organisation has definitely lost it's way, and has lost the respect of the public, and probably it's own staff. I think they need to reevaluate their purpose in life! But equally, society as a whole needs to get a grip; stop in-discriminant breeding, take responsibility for their own animals, and pop a couple of "common sense" pills when it comes to dealing with animal or wildlife issues - don't expect the RSPCA to be the savour of everything.
With any luck they won't be the R-SPCA for much longer.