I agree with this, especially for cases where a moderator has moved a comment discussion to chat. I agree with Cody Gray's comment: the whole point of a moderator moving comments to chat instead of outright deleting them is if they consider the comments valuable in some way, i.e. worth retaining in some archive, and that deleting them automatically after a period is antithetical to that.
A few days ago, I ran into this on the Aviation site: one of my posts had hit hot network questions, and the comments contained some valuable improvement suggestions, but many others began tangential discussion in the comments and a moderator moved the discussion to chat. As the post didn't garner enough comments for the system to consider the ensuing room worth retaining, it was automatically deleted. I felt that the comments were still valuable, even the ones with tangential discussion, so I flagged my post for a moderator to ask for the room to be undeleted, and the mod agreed.
Another reason I come across is when a user doesn't notice that a comment they've posted is a repost of a comment (from another user) that was moved to chat. While this can't by nature be wholly resolved (since the prior comment is behind a link), at least some users can and do check the chat room to see if they're reposting a moved comment, so having the room continue to be public will allow such users to check and allow others to flag new comments as reposts.
Finally, the current behavior results in confusion from new users. As an example, the 404 page combined with the prompt that one needs 20 reputation to talk can mislead them into thinking they need 20 reputation to read all rooms' chat transcripts. There's also the aforementioned issue of the point of chat moving being intended to archive comments, which auto-deleting the room nullifies.
An edge case I've also run into is where a question was closed with a custom comment, which was later moved to chat. In that case, it was fine as there were enough comments moved for the system to retain the room, but if there weren't, that custom explanation would have been completely wiped.
In conclusion, rooms that were created as a result of comment-to-chat migrations should always be considered "worth retaining" for the purposes of the system's rules, even if they don't meet the message criteria: they should be writable for 14 days, then frozen (which retains public transcript access), not deleted.
(I guess the only downside I can think of is users who object to not having the ability to delete their own comments once they get moved to chat - waiting for the room to go away on its own was proposed as a solution for such users - but there are other solutions, such as asking for one's messages to be anonymized.)