what makes you think so?
Reddit will no longer be searchable by search engines, and that will dry up any source of revenue that they could have gotten from those people who do not have an account.
People aren’t going to bother to sign up for an account just to read a Reddit article.
When you make yourself a completely walled-off society, you will slowly leak people who can’t be bothered to sign in every time. And you can no longer draw in people who do not currently have an account.
That’s a sure-fire recipe for ending yourself. Sooner or later.