An example closer to me: the search engine market is dominated by Elasticsearch, which is open source, extremely popular and is now even a public company. However, I decided to work on a small project called Typesense (https://github.com/typesense/typesense) to try and carve out a niche (and perhaps charge for it later).
I decided to take a dive simply because of the size of the market -- search is required in almost every SaaS application, and there are plenty of downsides to ES that can be addressed with a nimble, alternative search engine. And that's what I am doing :)