An open class is not like a nullable reference. It's not going to come back and bite you with an exception or boilerplate code.
An open class is also not entirely like a public method. It's not going to pollute your users' IDE code completion with methods they shouldn't care about or use.
Closing a class is a small case of control freakery. You already know that few people will have a desire to extend it, and certainly it won't happen by accident. Yet, you want to make dead sure they won't.
The legitimate reason to close a class is for security, and that may be a strong enough reason to close all classes by default.
Yet a more common, though not entirely legitimate, reason is to suspect your coworkers or users of being imbiciles who cannot be trusted. (Ok, to be fair, for certain projects with a complicated type heirarchy, there may be a similar argument as for public methods: coworkers may be confused on where the extension points are, and would see a benefit in the IDE's list of open classes.)
A third, completely illegitimate, reason is to simply ape C++ and C#, which made methods non-virtual by default in large part due to a belief that it would increase performance. Well, HotSpot showed them.
In sum, I don't think the points against outweigh the frequent and major benefit of the freedom of open-by-default classes.