I have no need for a Hashnode API but I have strong thoughts on paid vs free services, so I'll comment.
Programming communities are IMO tough to charge for - there is the obvious elephant in the industry, Stack Exchange / Stack Overflow, Server Fault and all the kids. It has been free and it probably will be free. It's a much more mature community and has how many thousands of active users.
Is that to say HN is worthless? No, of course not - the lights are on; we're here having a discussion and I think most people that post here, enjoy posting here.
If a service starts out free; IMO, it must always be free until such a time, a paid version is needed and the paid version offers me a compelling reason to spend money on it. Those features, worthy of cold hard credit (cause who accepts cash anymore?) will have different value to different people. You'll loose users for sure; some won't want to pay; some can't afford to pay; some will leave in spite. At my day job, 1 day their service was free, the next day it was $20 / year. They experience a 20% drop in traffic with the change (40k users). Over $20 / year! Ouch. They recovered over 2 years, but a drop none the less and now a change in the way they do business, what their customers expected and what they gave them.
Companies generally want people to share their content. Could you imagine if FB charged you to share to FB from your site? Few would do it. So to charge for API usage so people can embed HN's content on their own site, potentially driving more traffic to HN, seams like a really, bad idea.
A way of monetization that comes to mind is simply inject ads into the API feed. HN keeps the revenue from the ads, the programmer / website gets to display the feed and everyone (might be) is happy. Don't want ads in the feed, on your site? Pay for the API.
If I depend on a service, I'd rather pay yearly and save a buck or 2. If I'm not sure if I actually need your product, a trial period is in order. Those of us that have been here long enough know there is valuable content here - so selling the idea of an API wouldn't be hard. The hard part, is adding value so that it's a paid service.
Conversely - companies change the world every day for better or worse - re: dropping the headphone jack from iPhone 7 - so you roll the dice, try something new and see what happens. Maybe people are willing to pay for an API to embed a programming centric news feed in their site / app / whatever.