Will an S9 be able to pay for itself under its lifetime ?
I might be stupid, but when I count on it I come to the conclusion that an S9 is so expensive and produces to little so it wont be able to pay for itself for a very long time.
Definitely. IF it ever does.
And about "Correlation, but not causation", this is invalid as an action or occurrence can cause or be caused by another. So for anyone to drone out someone's idea of a relation between (for example) MUCH lower and longer block times vs 2 years of averages is like putting a towel over your head so the monstter can not see you...

Just like, if you step out of your house, you MIGHT get hit by a bus.. Or be pooped on by a bird.