Excel is great until suddenly it's not. When I started building ProjectionLab, it began as an excel spreadsheet that (at first) was easy to share and explain to friends and family who wanted a better retirement plan. But as they asked for more flexibility and new features, it spiraled out of control into a complex web of advanced Excel features that became opaque and unmaintainable. No doubt Excel is powerful and feature-rich, but sometimes layering on more and more of those advanced features isn't the answer.