It would be surprising to find that over the years any breed had not evolved from earlier types.
But this is precisely the point. There has been forced evolution towards ever more narrowly defined popular breed looks. There's good evidence from paintings spanning several hundred years that there was a fairly consistent small spaniel type if dog for a long, long time. Skulls, appearance and body shape of dogs before there were artificial 'breed standards' in the Victorian era, have for many breeds changed so massively in the next 100 years that the dogs would in many cases not be functional or able to reproduce by Victorian standards (evidence for that change was given in PDE in some detail). Some breeds now require medical interventions even to be able to reproduce. Without modern medicines for heart problems cavaliers would have even briefer lifespans. Because a very common heart medications, frusemide, helps with SM symptoms it is likely too that SM would have been spotted far earlier in the breed's history, on a more widespread basis, if so many adults hadn't been on this drug in their late adult years.
There has never been an argument that breeding for a dog in the breed standard weight is alone the 'cause' of SM, or that a larger cavalier from existing stock, all else being equal, would be any less likely to avoid it. This has been stated for years now by researchers.
There is however a good argument that changing the shape of the head and skull to further shorten muzzles and compress the skull shape for a head that is considered more attractive than earlier dog heads, has caused SM to worsen considerably in the breed.
There has definitely been a tipping point in the past 30 years and head development is now almost certainly the point at which the problem begins. This is being pinpointed in the foetal tissue research and was proposed by Rusbridge/Knowler and was suggested at the SM conferences in previous years on the basis that one of the suspect gene regions that came up on the initial genome scan work was that of genes responsible for skull development.
It is known that SM resulting from CSF flow/skull malformation is exclusive to toy breeds and seems more prevalent in short nosed, flatter faced breeds as well.
So while change might be expected (and there are many breeders who insist there is absolutely no change at all from earlier dogs, which seems extraordinary going from the old yearbooks and the handbooks I have), change is not necessarily a good thing. Especially not if that change has been predicated on choosing from an ever narrower collection of related stud dogs to achieve the current 'look'.
Breeding deliberately for even tinier dogs is not likely to help the situation. The fact is that larger breeds do not get this particular type of SM. So just being a toy breed is a risk factor, as several researchers have pointed out at the conferences. Breeding more 'normality' back into the skull shape seems like one potential course of action if there's going to be any realistic attempt to save the breed. Because while longtime breeders correctly note that 'the scratching disease' was around back in the 80s, it is now increasingly becoming 'the screaming disease'. That cannot be allowed to continue and it certainly is not going to reverse of its own accord. The problem has to do with breeding. Research needs total support to find out why, and breeders must work together to breed away from pain.
The KC had the option to review the CKCS breed standard and choose to do nothing that would help address any of this, not even to change the size standard which to all intents and purposes has changed anyway. How many 14lb cavaliers win in the show ring? How many show males even fit the breed weight standard any more?