Something always promised, never fully realized

I'm searching for a term or phrase for something which is always promised but never delivered. The way nuclear fusion is always 10 years away. Or how companies keep making jet packs, but never sell them.

Thanks!

Solution 1:

It depends on what nuance you want to give the phrase. A lot will depend on contest but, broadly:

"The search for the ever-elusive jet pack/fusion reactor." Gives the idea that there may be some hope for the product appearing at some time in the future

"The search for the mythical jet pack/fusion reactor." Somewhat dismissive in view of earlier attempts/claims/promises.

"The search for pie-in-the-sky jet pack/fusion reactor." Quite dismissive: the writer does not believe either will appear.

"The search for much promised jet pack/fusion reactor." More factual but cautious as to whether this version will work/sell.

"The search for the perpetually imminent jet pack/fusion reactor." Recognises that there have been many previous claims that something will appear in the very near future.

Solution 2:

blue sky

not grounded in the realities of the present

Merriam Webster

using the imagination to think of ideas that do not yet have practical uses or make money:

Cambridge

Sustainable nuclear fusion is "blue sky" thinking. It is visionary, but still beyond current technology.

Companies keep making jet packs, but never sell them. They don't make money on a visionary blue sky idea.