probably going to bury the "free" groups til they cant be seen in a feed, and promote the paid ones to the front page 100% of the time. much like how youtube does it, the most "paid" popular channels get 100% front page coverage.
That's assuming they don't look at a sub and just say "you're now paid, congrats" and the mods just go along with it. I can see greedy mods seeing the ability to profit from that kind of forceful hand and then complaining about disengagement.
That's how it always starts, then they'll widen it a bit more "Ok now we'll include subreddits that were created within 3 months of announcement" then a bit more "ok now within a year" then a bit more "ok now if it's a buy/sell subreddit with > 1M subs"
And then before you know it, you need to pay 1.99/month to access r/memes.
Oh but don't worry! I'm sure they'll come out with an "unlimited" tier that's 49.99/month!
It's not like a company would ever lie to the public for money. They definitely wouldn't ever think of slowly rolling back free services and monetizing them....... I mean where would anyone even get that idea about a tech company?
Sure, until they A- arbitrarily change that once people have gotten used to the new normal, or B- game the system and find an excuse to kill off profitable subreddits knowing the community will just turn around and spin off a new one... thus having to start requiring a subscription.