There are a handful of companies doing very well with models similar to what you’re describing. I can’t mention specific customers, but I see some of them doing very large scraping volume through our network.
It’s an industry where having a good product is more important than the amount you’re spending on marketing. If developers are happy with your product they’ll take it with them to future companies/projects and share it with colleagues.
It can be a cat-and-mouse development cycle where the sites you target break your functionality and you’ll have customers that will want fixes to be implement ASAP because they rely on your tools to make money.
I don’t know what you’re building exactly, but keep in mind there’s a good chance that you’ll need to commit to long-term, continuous, rapid development cycles if you want to retain customers.
Best of luck!