Presumably you could meet the boundary with "a dollah fifty in late fees at the public library" and find a way to push through from there. You'd have to find a way to publish or share your new knowledge. Studying at uni gives you access to experts in their own thing that likely have knowledge that could help you with your thing as well as a system designed to churn out these papers when you eventually find your thing.
Every day people discover new things but it takes attention, effort, and will to PROVE it's a new thing and more yet to share that with the world. Too bad you can't get an honorary PhD for doing that, at least not reliably.
Like the guy who found this somehow important new shape not to long ago? I don't think he has a PhD. But he did contribute. Not saying that it's easy though.
I've been making six figures while getting my PhD. There are plenty of opportunities to get your PhD funded if you are a US citizen. There are plenty more valid places to poke fun at pursuing a PhD but it is very common to have funding and thus no debt.