When do you imagine guns or butter will finally be obsolete?

Guns or butter? Probably never. A gun or some brand of butter? You do the math...

Just as LW is not "the online community", but just a (very specific) online cummunity. And it needs to cater to its audience, otherwise... It's luck is that it's just very well focused, we need to recognize this and make it more marketable.

Which is to say I don't think there is any such general rule of obsolescence in economics.

Technology is a big component in every model of economic growth, and by definition technological progress makes some other things obsolete.