I would second the remarks of the previous replies, and add that within SoPW, there are two other reasons why the sort of device you propose would not apply very well:

First, many SoPW nodes are created by AnonyMonk, so updates by the "owner" of the thread's root node are problematic.

Second, it's not uncommon for the "right answer" in a SoPW thread to end up being a combination of distinct contributions from different replies -- that is, no single reply contains the "complete" right answer, and to really understand a solution, a reader needs to track the thread linearly, which is what people normally do already.

(update: fixed grammar)