I'm looking for a word (or concise phrase) that describe a wish/desire that is too optimistic or demanding to be realistic.
Examples:
To hope that human beings will always remain rational is a ____.
A world free from scarcity seemed like a childish ____.
The phrase "pipe dream" used to be popular. Cambridge English Dictionary online defines it as "an idea or plan that is impossible or unlikely to happen."
Try fantasy. The word can be used to describe an unrealistic or improbable desire.
Merriam-Webster, definition 5:
the power or process of creating especially unrealistic or improbable mental images in response to psychological need
In your examples, the beliefs are unrealistic and can be supposed to come from some psychological need (a wish, a desire, a need for optimism).
Relevant synonyms include delusion (which has a stronger sense of false belief) and pipe dream (which is closer in meaning and often refers to plans).
You could try wishful thinking
The imagining of an unlikely future event or situation that you wish were possible.
There is also the informal idiom pie-in-the-sky.
A world where the rich don't get richer and the poor don't get poorer —that's just pie-in-the-sky.
This is referred to as Hope against hope:
to hope against hope [after Romans 4:18] : to hope where there are no reasonable grounds for doing so; to hope very much. Hence hope-against-hope.
You may also be hoping on a A wing and a prayer:
a wing and a prayer: a jocular form of reference (after quot. 1943) to an emergency landing by an aircraft; also fig. and as attributive.phr. in allusion to reliance on hope in desperate situations.
If you're more desperate than hopeful, you may also be Clutching at straws:
[Willing to t]ry any route to get out of a desperate situation, no matter how unlikely it is to succeed.
