: In Silentium, the Didact was captured by the Precursor, tortured, and
: released back to Forerunner space. The Precursor's plan was that the
: Didact would twist the Mantle around and fight the Flood in a way that
: hurts the Forerunner strategically and damns them morally. Whatever plan
: the Didact hatched, it should have looked reasonable and even inspired,
: but offer no hope of success.

: The Prometheans check that latter criteria off, but not the former.

: Thinking about Composing, I think it could have fit into the Primordial's
: plans. Maybe organic-derived AI (Rare by Forerunner standards. They have
: much better ways of seeding AI) seemed like the best of both worlds,
: immune to the Logic Plague and incapable of being infected by the Flood.
: The obvious route is to tailor organic-derived AI to make them perfect for
: combat and craft an appropriate vessel.

: The problem is, the immunity of organic-derived AI was a lie, just like the
: cure to the shaping sickness. It's merely a facet of the Precursor's
: overall plan to make the Forerunner's last stand as painful and degrading
: as possible. The tweaks the Didact makes to the AIs will only make their
: suffering worse.

: All that's needed to make that work is a weapon system that poses a credible
: threat to a galaxy-sized Flood infestation. Prometheans and Borg-designed
: armor ain't it.

Well, I never said that the Von Neumann solution had to be really workable, just something that looks feasible on paper. The Didact is essentially a pawn for the Gravemind, but from what we see he's not supposed to be completely bananas. To think of him as just madly obsessed and backed to a corner gives much more weight to his character than just making him utterly and incomprehensibly irrational.

Like you said the Prometheans should look like something a strategic mastermind like him could reasonably concoct as an absolute last resort, the kind of plan the Forerunners would've never entertained in their prime because of all the moral baggage associated with the Mantle. And it would provide an ironic mirror to the Flood with the whole composition and hegemonizing swarm thing (you could have Promethean ships armed with Composers sweeping sentient-inhabited planets for more "souls" before the planets themselves are harvested for raw materials).