I do half like that thought as I know exactly what you are saying as I have been asked to perform an edit when their time limit had expired (other software but the principle is the same) and the reason was 100% valid to request the small edit but my only concern here is said content might of been quoted in other threads or comments on resources/media perhaps via copy/paste etc so it could be quite potentially resource intensive to 'search everything' to fix this.
The obvious answer is to simply only update the thread in question even if its huge its less than the potential of my 'caveat' above I suppose. Perhaps a checkbox to give the "mod editor" the choice of this maybe.
As I say I can clearly see why you are suggesting this just wanted to add my little bit to it.