What would be the point? Reddit doesn’t make any content. They’re just a platform. If they go ahead and paywall subs, those subs are going to have a tiny potential subscriber base. Therefore, they will be less attractive to post to (smaller audience, fewer upvotes etc).
About the only place I can maybe see it working is AskHistorians. And you pay the Historians to answer the questions. Which would of course reduce the amount Reddit takes from the paywall. Doesn’t seem worth it, to me.
Even then, I think the Historians would rather reply in a new free sub with wider readership than take $20 for putting in three hours of work responding to something. They do it because they’re passionate. Not for money.
The problem with this is trust. If you could seamlessly migrate like this, there’s nothing to stop someone faking a long post/comment on their own instance, making them look very legitimate and then migrating that account to a trusted/legitimate instance.
Then using that for spam/selling block chain etc.
People are the reason we can’t have nice things.