If every startup could work with one customer from the idea stage through prototype and beta, so many problems would be averted. And if that one customer leads you down a narrow path, it is far easier to broaden the scope of useful product than pivot an unwanted idea.
This sounds like a perfectly legitimate way to earn a living, and more mature and wise than chasing some unvalidated unicorn dream and running out of cash after a year of exhilarating misery.
Just keep growing your business sideways, try to reuse as much as possible of your IP and experiences.
You are on the right track
However, I do think it's worth considering the value-add having 'smart' money can bring——networks, legal advice, business experience, or other forms of expertise. Granted you have to be careful about gauging it right before accepting the money, but it might be right for the founder's vision and business opportunity.
That is lose control of your company, take in money faster than you can use it and lose it all with over 90% probability.
As mentioned by `icu` in the other response, smart money can be great. Just make sure it's SMART and not SF-herd mentality BS.
it sounds like a horrible business, resting on its laurels.