Why did you pick 0.0005 BTC ? That is a mostly arbitrary number.
I estimate a current-worst-case "orphan cost" of an average 250-byte transaction is 0.00008 BTC. See https://gist.github.com/gavinandresen/5044482 (more accurate analyses welcome, I don't pretend to have a monopoly on the "right" answer).
That number will drop as CPUs get faster/cheaper, or bitcoin value rises. So you could argue that even though dust is not economical to spend today, in 20 years it will be.
So I guess I'll rephrase my question again: Rough, back-of-the-envelope: how much does it cost to keep a dust-like transaction output in the unspent outputs set for 20 years?
If it is a lot, then we should set the "expected time when it will be economical to spend" to either "right now" or "very soon."
If it is tiny, then we shouldn't worry so much about optimizing unspent txout size, and concentrate on other things.
I have no idea what the answer is.