Suppose you start a product or a service in a niche, where there already are a few players that dominate that market.
How would you convince users to start using and paying for your product/service?
The first step I'd say would be finding a pain point and solving it for them. No one will pay for something if it doesn't add significant value compared to what they're already using
I think this is exactly right.
The good news is that your competitors have established that a market exists.
You just need to position yourself as an alternative to them in a powerful way. This can be based on emotion, simplicity, or being strong where a competitor is weak.
Depending on the market, you could also market to a slightly adjacent space, solving the problem differently for a slightly different set of users. Owning a use-case can also help amplify network effects.
You need to figure out what your UVP is, aka find out what makes your product different from competitors'. The easiest was to do that is by talking to people in that niche - it may seem hard, but its actually the quickest way to get straightforward answers from potential users. Ask them about their problems, what they like/dislike about competitors, what they think about your product, etc.
Once you know your UVP, you can use this to market your product/service.