10,000 post limit too low

[DeanMarkTaylor (Dean Taylor)]

This feature is there for a reason, refer to this topic where the feature was introduced:

Using private / personal messages as long term chat

One thing we’ve noticed come up on a few sites is a pattern of behavior where users make almost no public posts, but spend most of their time in very, very long PM topics that can be tens of thousands of replies long There are a few problems with this: We currently have a massive perf problem with longer topics as we send down a list of all IDs of all replies in the topic, which can be enormous – if the topic is 50k replies long, it includes 50k ids in a list in every request. Longer term…

Note this “auto closing” helps keep Discourse performance high.

The setting you might be interested in is:
“auto close topics post count”