Word or phrase for an idea that is good in theory but not practice

A contribution to the spectrum of meaning that you refer to is Pipe dream.

Pipe dream (noun): an idea or plan that is impossible or very unlikely to happen.

Example: Her plans are not realistic - they'll never be more than a pipe dream.

[Cambridge English Dictionary]

That brings to mind pie in the sky, castles in the sky, fool’s paradise, or even a chimera.

Chimera: a hope or dream that is very unlikely ever to come true.

Example: Why do people chase the chimera of fame?

[Cambridge English Dictionary]

I imagine that there are theories without practical applications, and, versa, as with the futility of asking about the reason we exist, but, how can a good theory not jibe with its practical application(s)? Don't we have to, in practice, rigorously check out theoretical predictions, etc, before we decide, how good are the theories?

How about, a good try at a theory? A non-working theory, like String Theory, which, as far as I know, never predicted anything that could be physically checked out? Or paradoxical theory? Relativity Theory, and Quantum Theory are viable theories, in practice, but, continue to appear to, I guess, mostly disagree with each other.

For something to be good in theory, but not also in practice, leads me to question either, or both.

I find myself hesitating to suggest this word because the most applicable definition appears in Wiktionary which is not the most authoritative of online resources. However the word is, if you accept their second definition, perfect.

The word is impracticality which is defined in most dictionaries as being an abstract, uncountable noun but is definded in Wiktionary as:

impracticality (countable and uncountable, plural impracticalities)

1. The state or quality of being impractical

2. Something which is impractical.

The Oxford Learners Dictionary says that impracticality can be used as both a countable and an uncountable noun and says that its plural is impracticalities but does not offer examples of its use as a countable noun.

The Wiktionary entry suggests that it should be possible to say things like

"The proposal is attractive but is, unfortunately, an impracticality.

However I have only seen the plural used to express the fact that there are multiple ways in which something is impractical by saying things like.

Unfortunately the proposal suffers from many impracticalities

I offer 'impracticality' as a possibility but only if you accept the second Wiktionary definition.