This is why I get annoyed with the current trend against doing technically hard things as startups. The up-front risk of your thing not working is much higher, but if it does (and there's a market) then you're in a much more defensible position. You're basically front-loading risk. And since the thing you're risking is time, front-loading is something you should do if you can.

There is the problem of telling whether there's a market for a given thing before you invest six months or a year building it, so I'm not advocating sinking a whole year without shipping, but there's definitely merit to starting with something that can't be built as a rails app.