I think you're going to know by one metric. Quality of content over the next ~3 - 6 months. Whether subs stay or go is one thing, that's been part of Reddit for the 12 years I used it. What would get folks to leave is when the communities they are interested in aren't supplying content.
So if you lose some lurkers, that's not gonna matter because they didn't post anyways. If you start losing power users, who regularly feed your community content, what's going to drive you to stick around? If you ask me, I think the fact we are even having this conversation means Reddit is losing in this equation.
What gives me lots of hope from this is then saying this is similar in size to outer worlds. Being more focused and compact the story shines through. Something more open world games should focus on rather than padding game time.