Utopian
Etymology
From "Utopia," the imaginary island in Thomas More’s 1516 book, coined from Greek "ou" (not) and "topos" (place) — literally "no-place." More built in a pun: "Utopia" sounds identical to "Eutopia," "good-place," so the ideal society is, by its own name, both the best place and nowhere at all.
adjective · /yoo-TOH-pee-un/
Aspiring to an ideal state of society, often to a degree that seems impractical.
What it names
A mode of thinking that pictures perfection — useful as a compass, treacherous as a blueprint.
Example
The plan was utopian in the worst sense — it assumed everyone involved would behave perfectly and forever.
Related idea
Utopian describes visions of an ideal order; quixotic describes the temperament that chases such visions gallantly and impractically, named for Don Quixote tilting at windmills.