At least to some extent, if moving to chat killed a discussion, it was probably for the better, e.g., because it was going in circles, dying anyway, or the disputants had other motives than the discussion itself.
Alternative explanation why moving to chat kills discussion
I guess this is because people hate chat, but I have no evidence of that other than people not using it.
This implicitly assumes that people post comments because they truly care about the discussion for its own sake, helping somebody, making the world a better place, etc.
But users also post comments:
- to have the last (moderately visible) word,
- because they cannot ignore a notification,
- because they think their comment is very important (which is quite often an blatantly overoptimistic),
- to spread their pet theory, view of the world, …,
- because they want comment upvotes for badges or their self-esteem,
- to troll.
It’s hard to estimate to how much this amounts, but considering the following observations, I expect it to be quite considerable:
There are some users who actually consider it important that comment upvotes are lost when moving to chat or argue with the scores of their comments against them being moved to chat.
Users rarely ever take the system’s offer to take comments to chat for them (that they must see in a fair amount of cases, where comments are eventually moved to chat by the moderators), even though it would give them more control and their previous comments have a higher chance to survive.
If the comment thread moved to chat was no discussion but a bunch of independent comments (usually a series of answers in comments and similar stuff on questions), many people just continue posting such comments, even though an asker who really cares about the issue would probably look at the chat too (and thus see potential answers in comments). There is quite some overlap to those who will not continue comment conversations in chat.
Stack Exchange’s gamification model has a tendency to attract such users.
If moderators move comments to chat, users who mainly post comments for the above reasons have no reason to continue the discussion, which then dies.
The impact of such users is exacerbated by the fact that a discussion needs at least two people to survive.
So if we consider discussions between two random users as a rough model for moving to chat and half of the users lack the motivation to go to chat, three quarters of the conversations die.
Moreover, people who do not continue discussions in chat for the above reasons are more prone to have discussions that attract the attention of moderators in the first place, because they do not know when to stop.
The latter need not even be a function of the users, but of the comment threads in question:
Many discussions I move to chat have already reached the point where everything relevant has been said and the disputants either bicker about details of details, are not willing to give in at all (“you are wrong”; “no, you are wrong”; …), or have completely digressed.
These discussions are already on the life-support of the will to have the last word.
When such a discussion is moved to chat, it is often a welcome opportunity for at least one of the disputants to leave the arena without losing face.
And now, to your questions:
whether others agree with this assessment
Yes, I do.
whether others think of this as a bug ("we should encourage off-topic discussion, in the right place") or a feature ("we should discourage off-topic discussion")
If my above explanation is correct, I would consider it a feature, however as a mercy kill of unhealthy discussions rather than as a suppression of off-topic discussions.
However, my explanation above is just an educated guess – more data would be great.
if "bug", what we should do about it.
If my suspicions above are correct, a contra-intuitive conclusion would be to move comments to chat earlier in some situations, i.e., before the discussion has reached the point where at least one party does not want to continue it. I am not sure whether this would be overall beneficial though.