How would it change the UX (or UI) at all? Question and comment "scores" constantly change as they get upvoted (and occasionally downvoted). This would just be another factor in determining the present score.
As far as "theory" questions, such a designation could simply be another moderator privilege -- high-rep individuals would have the ability to vote to prevent a question from decaying. Perhaps an additional requirement would be that the question must have been transferred to Community Wiki.
You haven't explained why "this would be terrible UX", and it would be fairly trivial to add a special "theory" tag or something more general (maybe restricted to high-rep users or mods) to avoid the problem of unwanted decay.
> Because it seems obvious that watching one's hard-earned score shrink would be a bad experience.
But it's a Q&A site, not a point-gathering exercise. Leave the displayed score of the answers/questions but decay the benefit the author gains from it.
Indeed, but if you've put in a lot of work asking/answering good questions and they reward you with standing then it sucks to suddenly start losing that. There is already a certain level of inflation in that those who are continuously active will keep raising the bar. I am rather proud of my contributions and score on SO and don't want to see that go away
