Psychological factors and dispositions such as optimism and pessimism have little to do with public policy, but the typical response to people saying, "these policies are unsustainable" is simply to say, "don't be so pessimistic". This response is nonsense.
For a brilliant discussion of the "this time it's different" syndrome in the financial realm, see the book,
This Time It's Different: Eight Centuries of Financial Folly.
Alas, we are living in times of political, religious and ecological folly as well.