One of the aspects of top class racing is the level of drop out by winners of Group or Grade 1 races. I remember writing to Lord Oaksey in his Telegraph journalist days about 60 years ago pointing out how few Cheltenham winners or placed horses ran in the following season and nothing has changed.
Consider Faugheen, Douvan and Thistlecrack, Jetski at the top level. They might win another race or 2 but never reach the same level as on the big day. That is why horses like Arkle, Best Mate, Big Bucks, Bula etc. were so exceptional as they kept coming back at the same level year after year.