Reddit doesn't have to close in order for this blackout to be successful.
Myspace is still alive but not relevant.
I think the best-case scenerio is that the blackout, combined with 3rd party apps shutting down will cause a partial migration to fediverse-based solutions.
As long as the communities get a kickstart then it will change the companies trajectory.
In the /r/apollodev post there is a section with all the info of the latest call they had before shit hit the fan.
In that, Reddit said that "[Reddit] is open to discussion, if moderators promise to keep subreddits open", so personally, I don't think it won't change anything.