And those NSFW posts are far from just porn. It's frequently news (especially related to war), the "vice" subreddits (cigars, guns), mental health subreddits, ect.
Also, 4x of Apollo Ultra is probably not enough. After the Appstore takes its cut and the dev pays himself, you're not left with enough money to pay for the API access of the power users you're now inevitably left with. Powerusers, who, again, can't access 40% of posts on your app.
And for the longest time, many subs based their flairs around the NSFW tag combined with some CSS hacks, especially for subs based around TV shows wanting to add spoiler warnings. Some subs never bothered to update to the new system and still use this.