As someone who displayed military aircraft occasionally and in a fairly haphazard way over a period of over a decade, nothing could be further from the truth, no matter how it might be seen from the outside.
I have seen/displayed aircraft that did not have a routine planned, let alone a display auth or anything like it.
Nowadays is much much stricter.
The reason that a Shoreham has not happened before is because it is incredibly unlikely to happen, not because of a dropping in display standards.
There is no decline.
Displays have never been safer, but the current hysteria about safety is accelerating faster than safety can keep up.