I'm a bit hesitant to support the star system because of the existence of noobs/trolls/illiterates, but with this community I doubt that'll be a problem, and even if it would then the problem would be just as prevalent in poll voting. So we'll just have to try it out and see.
I approve of the change from archiving to demoting. IMO it is currently way too easy for strong ideas to be archived, and then way too difficult for ideas still up-to-date to be revived in a meaningful manner.
As OldTrees said, outflow must equal inflow. We cannot allow leniency (in outflow) without enforcing strictness (in inflow).
Suggestion #1:
Instead of having a set point value your card idea thread rating has to exceed (e.g. 3.99) in order to move from the Smithy to the Crucible, allow the Crucible to hold only X highest rated threads.
Specifically, after the cards with the lowest Crucible poll votes are kicked out of the Crucible, look at the Crucible card with the lowest thread rating. Then, accept up to X cards in the Crucible submission thread, but only the ones with the highest thread rating, and only if that rating is stronger than the weakest card currently in the Crucible.
This will not only keep the Crucible from being bloated, but also maintain an implicit standard of quality for cards moving through the system.
As I said before, demotion > archiving. That's why I would also suggest disabling the Crucible Archive.
Suggestion #2:
Instead of sending cards that leave the Crucible into the Crucible Archive, send them back into the Smithy, so they have another chance of getting into the Crucible if the card is good enough. Of course, there should be a long delay from when you leave the Crucible before you can resubmit your card for approval into the Crucible, say 2 rotations.
If this mechanic is implemented in tandem with the mechanic in Suggestion #1, then we can give card ideas who are demoted due to poor circumstance a second chance without prioritizing them over fresh, new card ideas that outrank them in thread rating.