All of these are good questions, and ones that many who love the breed, especially many good breeders, consider all the time.
I would start by saying that ironically, you named two other breeds with serious health issues too! -- dalmations and goldens. Pedigree dogs by their very nature have narrow gene pools and almost ALL breeds would never survive on their own because even though there may be millions of dogs of that type, they often have a genetic diversity of only a hundred or fewer individual dogs and thus they are way, WAY below what is considered sustainable for genetic diversity in the wild. So just about ALL purebred dogs would go extinct if we humans didn't maintain artificial breeding programmmes!!
If you watch Pedigree Dogs Exposed, the BBC documentary (you can find it on YouTube, or buy the DVD) you will see many of these issues explored, and cavaliers are highlighted (rightly) as one of the more problematical breeds. Therefore, many of us here feel that perhaps health testing should be mandatory for breeding and showing dogs as a result of these issues.
Many of us get this breed because we wish to support rescue and have rescue cavaliers, or to support the good breeders who are trying hard to reduce the incidence of these conditions. IF you work with a truly health-focused and testing breeder, you can reduce significantly the chances of either of these conditions causing a cavalier problems during its lifetime.
Cavaliers for us raise significant worries though because the common health issues in the breed are sadly very serious and can bring terrible pain.
The comparison to humans with 3 legs is actually a false comparison because humans regularly and knowingly 'breed' when quite aware that their children risk serious diseases that run in families -- higher rates of cancer, Parkinson's, syringomyelia, MS, severe conditions of all types. Humans do not 'die out' because there's billions of us and a massive general genepool that can easily absorb these deaths (if you wish to look at it from a quite coldly biological point of view). Also we generally feel people have the right to make their own decisions as to whether to have children and potentially pass along such risks. We also no doubt find the issues less obvious or urgent because many of these conditions only affect people when they are older adults. For people they do indeed form serious philosophical and moral issues -- REAL disease and REAL risk of pain, not three legs...
Dogs have a far shorter life and also we humans make the breeding decisions thus a single poor breeding decision made by a breeder or a pet owner who thinks they want 'just one litter' etc, can have wide and detrimental impact on the breed as well as all the puppies born from that mating.
. Here on CavalierTalk we strongly advocate working with reputable breeders who do the proper testing, and we encourage all puppy buyers to research the breed to make sure they are ready to take the risk of owning a breed that can have such problems -- and to only please ever support good breeders. Therefore we offer puppy buying guides, links to good websites that give further information on choosing a cavalier breeder and on breed health issues. We raise funds for research into the problems in the breed (which is supplying valuable information to researchers on the human forms of the same diseases). We lobby and educate.
Anyoie is very foolish though if they think they will easily find any purebred breed that doesn't have a long list of potential genetic issues that can compromise their lives -- or that are bred with physical features that we humans find 'cute' or attractive but can cause anything from discomfort to devastating pain and illness for the dog.
Take pugs (also featured in Pedigree Dogs Exposed). I would far rather deal with cavalier potential health issues than support the current appearance of the pug which directly causes many serious problems and means ALL pugs lead a life compromised by their appearance at least to some extent! The wrinkled face can cause painful skin irritations between the folds which need to be kept clean (hardly something that breed could do on its own!). The flat face creates breathing problems for about 100% of the breed, many of which so struggle for breath that the breed is known for seeming to fall asleep while sitting upright (YouTube is full of videos of what owners think is a 'cute' sleeping behaviour! It is because their dogs cannot get enough oxygen and thus fall asleep -- and often sleep in odd positions to open up the breathing passages). Pugs have such shallow skulls that eyeballs are actually at risk of falling out of the sockets. The large exposed eyes lead to eye problems and infections and a high rate of a painful condition in which their eyelashes grow in the wrong direction and scratch their eyeballs and need surgical correction. That 'cute' corkscrew tail is linked to a gene that regularly causes their spines to be deformed. Many of these problems need surgical intervention for the dog to even be comfortable.
Yet many owners and breeders insist they are happy and cute and the snoring and gasping breath is just 'normal'. You'd be hard pressed to find a single vet or canine researcher who would say the same.
Many of these breeds did not look this extreme 50 to 100 years ago (pugs had a snout and normal eyes, to start with, and didn't have corkscrew tails). We humans have made these decisions for them.
Most of us here would rather work toward pedigree dog health, not abandon the breed or pedigree dogs. We support researchers and we argue that far greater responsibility is needed in breeding and showing, as well as protections for puppy buyers (which would be the single fastest way to get breeders to start to work toward health -- if they had some cost liability if it were shown they failed to test their breeding pair for the illnesses suffered by the offspring. You can't eliminate anything totally but you can reduce incidence and if a breeder isn't testing they are breeding with no knowledge at all of the state of their breeding stock when they have internal problems that are not obvious in the outer appearance of the dog).
I would be the first to encourage anyone who is concerned about cavalier health problems to consider another breed immediately. But I would also be the first to say: you will have a hard search and a very narrow range of breeds to select from if you think you can find a purebred dog that doesn't have a menu of potential genetic problems and/or physical problems caused by our desire for a dog that looks a certain way. The most healthy choice for a dog tends to be a mixed breed -- as demonstrated by the fact that insurance premiums are lower for them as they have fewer illnesses (NOT 'designer crosses' however which can have all the problems of the two parents breeds if the health history and tests are not known for the parents!).
Anyone who truly cares about dogs will be concerned about all these issues. And we are.