GitLab, Mattermost, RocketChat would be a few good examples of products that began as OSS alternatives to well-established players.

Outline (https://www.getoutline.com) is successful on most metrics you'd judge a business and OSS project on.

I'm not sure if any of these ever pitched themselves explicitly that way on HN though. I think explicitly labeling as an "Open source alternative" brings a lot of baggage and expectations, and often says more about the maintainers philosophical POV than ability to create a sustainable business ;)