As for the fact that this wouldn't be retroactive... who cares? The benefit of doing it would be achieved even if it weren't retroactive (since the benefit applies to active threads).
Furthermore, there's no need to do any development... It could simply begin as convention. It might take some time to get the "critical mass" behind the convention, but if a good number of monks really started doing this, it might just catch.
If at some point in the (distant?) future, we all decided that there should be some sort of deliberate functionality around it, and someone has the time and energy and ability to code it... great! But if that never happens, or doesn't happen for a long time, it doesn't really change anything. The analog that comes to my mind is how we gradually mutated from the (default) title of replies to replies changing from: "Re: Re: title" to "Re^2: title", or "Re:*2 title", or various similar things by convention (certainly not uniformly applied, but still widely applied), until eventually someone said: screw it, we should just code it (and did). It wasn't retroactive, it was no big deal, but it was still cool.
Anyway, I guess I see it as:
- does no harm
- does some good
- is easy to start doing
- it's an extensible path toward future (maybe better) stuff
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