Clumsy language. I suggest:
1. Was decades in the making: took decades to make; is now made.
2.Has been decades in the making: started decades ago; not yet made.
An even clumsier reply, Centrox.
1. Your result is not the only alternative.
The huge project was decades in the making but it was never completed.
You didn't even answer the original question. A clumsy attempt overall, Centrox.
2. As regards this one, the project could have been abandoned, and the speaker is using the PP of the HOT TOPIC/the PP of
Has been decades in the making: started decades ago; not yet made.
The now abandoned project has been decades in the making,
started decades ago, not yet made/completed and it is no longer being worked on.