I mean, the way they succeed with Starship is failing over and over again until they don't anymore. In that sense it is (now, not before SpaceX was scaled up) solely an issue of funding: will it work before the company runs out of cash?

At this point, I imagine most every investor in Earth would be tripping over their own legs to buy SpaceX stock if they could. If they ever run low, their ability to raise more money is as sure a thing as anything in this world these days. Additionally, Starlink is going to be an absolute revenue monster very soon, even if Starship for some reason ends up taking another decade to get working.

The only way Starship fails is if SpaceX goes broke. SpaceX is not going to go broke. Therefore, Starship will eventually work, after some variable and as yet indeterminate number of prototype failures.