Important for HNers who don't understand this already to internalize this: "VCs will try to get their money back" by selling the company, not by taking money out of the company. Once you're talking about these sums of money, the odds-on form the sale needs to take is an IPO.

Assuming a company continues to grow quickly and looks like it has a good shot of IPOing in the forseeable future, there is no sum imaginable of money which cannot be found for it. (That is a risk factor for every startup on the IPO path, because nobody grows forever and each additional financing round dilutes you more, but $100 million is not, in the grand scheme of things, an impossibly huge amount of money to use to build a software business.)