There are some special cases. If someone thinks his life is worthless if he doesn’t have something that could be bought or done with a $1,000,000, then the gamble could be justified. The thing that he buys pumps up the utility so much that it’s more than thousands times the utility of $1000. But this is probably a really rare case.
Diminishing marginal returns.
The utility of your gaining a $1,000,000 is not a thousand times the disutility of your losing $1000.
There are some special cases. If someone thinks his life is worthless if he doesn’t have something that could be bought or done with a $1,000,000, then the gamble could be justified. The thing that he buys pumps up the utility so much that it’s more than thousands times the utility of $1000. But this is probably a really rare case.
Medical issues that make life miserable but can be fixed with ~1M$ would be a (bit more concrete) example. Relatively rare, as you said.
If losing $1000 does not impact your quality of life, your statement appears to be false. Am I missing something?
ETA: It is obvious that the returns are diminishing under certain assumptions, but not that they diminish significantly.