OK, so if each user were to use their own API key, reddit would still have the same costs but nowhere near the revenue, which speaks to a pricing structure aimed at the status quo where the API key is per developer instead. What's the betting that if apps switch to allowing users to specify their own API key that reddit actually allows most users to do this? The '90%' number is meaningless: apps which use the reddit API (or any API with a free tier) have an extremely long tail of niche uses which effectively no-one cares about.
The comment about API optimization is inane, apps already try to do this and bump up against the problem that reddit's API is atrocious (try actually getting all the comments in a comment chain once they get collapsed! if you actually want to succeed, even on a chain with only ~100 comments, you're gonna need around 30 API calls. Most apps don't bother to try, and the 'more comments' button is invariably broken).