Ask HN: How to create a service based on crowd sourced data from scratch?

I am trying to create an alternative to a very annoying website that I used to love in past, but now is filled with anti-patterns. It provides crowd sourced data to the public and to see that data you have to signup and contribute to their database first. Even after signing up, it keeps asking for premium membership after a few clicks of mouse.

I want to create a more open, more accessible and least annoying alternative for that website.

My concern is how do I get over the chicken and an egg situation. The website I mentioned above gets traffic because it has some data, and users are also contributors to the data source.

If I create similar service, I won't have any data (I don't know if web scrapping is legal to populate initial data), I won't have any users to contribute to the data source.

If all goes according to planned, I am also willing to provide paid tier with better services.

Please help me with your suggestions.