Again, this depends on the area.
Where I live there are many rinks and many kids who are training for Regionals and hope to make it to Sectionals and maybe even Nationals someday. It's true that a few skaters here and there pick themselves up and move somewhere else, usually SoCal, to get a "better" coach. The vast majority do not. And the vast majority of coaches here do not have a reputation at the level you are talking about. They are still pretty busy and still make a good living. A few of them do manage to get skaters to Nationals here and there but most likely you've never heard of them or their skaters. Those coaches aren't necessarily any busier than the ones who do not get kids to Nationals nor the ones who do that and you have heard of.
Remember: Karen Chen learned to skate here and didn't move to SoCal and Tammy Gambil until she was quite successful and a known skater. The same with Alyssa Liu, who actually still trains here but with coaches with more of a history similar to John and Silvia.
Therefore, I don't believe that John and Silvia have to have elite skaters in their stable to make it as coaches. For one thing, people know they have already had that kind of success. But even if they hadn't, there are enough serious, but not known skaters who would be happy to take lessons from them and have no plans to move away to a coach with elite skaters.