Excellent question, MJT.
I think the answer to your question lies in the concept of Ma'at itself, and how the weighing of the heart ceremony relates to the choices we make as mortals here in this lifetime.
In order to make it into the Duat, the Egyptian afterlife, one had to go through the "weighing of the heart" ceremony, pictured in the masthead of Ma'at's website above.
If the heart being weighed was found to be "as light as a feather", then the owner of the heart was allowed to pass through the gates to the Duat and his immortal existence was somewhat guaranteed. (yes, he still had further tests to pass, but this was the first biggie he had to get through).
Now, if you had a revelation that told you how to build your tomb, or a temple, or a civic project of some proportion, you would be honor-bound to commence that project. Unlike our modern mindset, however, the road to the Duat
is paved with good intentions. Starting the project was seen, probably, as good as actually finishing it, when the gods called you up for judgment time. Not starting it, or not doing your best to complete it once started, would probably leave your heart feeling a bit too heavy and you'd end up getting it thrown to the nasty beasty that sits behind Anubis.
If we drop back away from pyramids and into the realm of mastabas, there is quite a bit of acceptance of the idea that the mastaba represented the Primeval Mound. If, in the case of a pyramid being unfinished and completed as a giant flat-topped square, we can imagine that they would have considered it to be representative of the same Primeval Mound that had served kings well for centuries before the pyramid was invented. It's kind of a "drop back and punt" option, but there's little reason to think it wouldn't have been an acceptable response to an early demise by a king.
Those are my culturally specific thoughts on the matter. I think you'll find Lehner's discussion of the evolution from mastaba to pyramid to offer some nice insights into this as well (TCP, chapter 2 or 3, I think).
Anthony
You can lead a horse to water but you can't make him think.