Uh... Flash was a genuine firehose of security flaws. I mean, yeah, they patched them. So "battle tested security layer" isn't wrong in a technical sense. But, yikes, no.
No, Flash was terrible and killing it was good.
Honestly I think a lot of the Flash mania is just middle aged nerds fondly remembering their youth. The actual tool was a flash in the pan, and part of a much more complicated history of online content production. And the world is doing just fine without it.
I remember my teenage friends creating things with flash in a way that doesn't happen on the modern web.