Their hosting costs are not going to majorly change due to API access. By their own accounts, less than 5% of users access the site via the API and if anything, those would impose less costs than someone needing to load the entire UI alongside the API calls (albeit using a new GraphQL API that I’m sure is more efficient but not available to 3rd party apps).

API hosting is a drop in the bucket, even if their own accounts are to be believed wholesale.