@wordshaper I think a subscription-or-ads model for the whole site would be fine, a la YouTube Premium. But paywalling subreddits I feel conveys a “monetize everything” incentive to subreddit mods that will lead to the YouTube-ification of subreddits in producing high-engagement content at the expense of quality.
IMO anything you can do with paywalled subreddits, you can already do with private subreddits today, without the pressure to monetize.