Which ones would they paywall? The ones which would be the hardest sell for them would be the big meme subreddits, given how many other sites can be searched for memes.
I could potentially see them having some success paywalling all of the sub’s where technical questions are asked looking for solutions to particular problems (think r/DIY). But then again you wouldn’t pay just to have 1 question answered, and would generally just try a different forum if it could be answered elsewhere.