Most comments should end up as an edit to the question or answer, clarifying some point, at which point they can be removed. The Question and Answer are the important things on Stack Overflow, not the communication that went into creating and tuning them.

The remaining comments are just fluff, such as "thanks", which can more appropriately (for the site) be expressed as an upvote or accept. Or they're asking a new question, in which case the parent's comment applies.

This culture doesn't really invite towards answers improving over the years as more and more people who might know more happen to come across them.

Interesting how this is the case, because that was one of the key goals of the site when they were first building it.

I listened to Joel and Jeff talk about it in their early podcasts when they were first building the site. To be the definitive answer for a question requires that the answer be able to change and evolve over time as new information becomes available.

That's what the edit link on answers is for.