Currently there's a short window of time in which you can update your title, mainly for typos and such. I know this isn't necessarily what you're asking about, but I feel obligated to mention it. :P
The issue with this is that it could also be abused by people that just want to keep their posts looking updated without actually offering new content or any value. It also makes moderators' jobs harder when people can screw around with their titles and make them offensive or otherwise not fitting to the subcommunity.
Forums are relatively transient, and in this particular platform the goal is for conversations to end when interest fades rather than persisting forever on a circlejerk (depending on the sub-community). Title changes would end up being somewhat unnecessary since threads move past the public eye once interest fades unless they are in Best, in my opinion.
This all being said, I think it heavily depends on the subcommunity. In a fanfiction setting it might be reasonable, though the issue is that editing a title doesn't bring the thread back into the forefront. You can edit the title all you want, but the only thing that'll allow it to be seen is continuous community interest.