I am sure this will just be the start and there will be other breeds to come, so many which now seem to be so far away from a healthy population. French Bulldogs, Pugs surely next, for popularity, but others to come.
Seems a huge shame for Cavaliers, which are such a popular pet. I really wanted a Cavalier, but just couldn't bring myself to buy one as so few breeders are doing all the health testing needed, you'd be forever waiting (years, not talking 'oh I want it now') for a pup with sufficient testing. I hope breeders do heed this warning and get on top of it fast. It does seem that there would need to be some outcrossing to bring it back.
That said - there are plenty of registered breeds that are very healthy with years of science applied to health testing to get the breed in a very good position - and in no way should people assume that poorly bred designer crossbreeds are in any way the answer.