A wide portion of the Internet is not ad supported. Consider a user searching for product support: the company benefits from the user self-solving the issue without having a ticket filed.

This, may, however be to the detrimental of discussion platforms, but if that drives people to self hosted discussion platforms instead of relying on reddit, then it is a net positive.

Discourse's most recent blog delves into this and is an interesting read, I will submit that I think.