It leaves out a large part of the problem of selling something. How will enough interested people discover the "simple web app"? If there are competitors (chances are there will be) and the app's page is in the 12th results page of Google and PPC keywords cost $5 per click, getting enough relevant traffic to ultimately reach 400 subscribers can be quite a lot of hard (and costly) work.

As an indication, if you look at point 5 in this survey http://webappsurvey2009.techcrunch.com/ you will see that ~50% of marketing sites of web apps got 0-1,000 visitors a month.

It can be done but don't just expect to "build it and they will come."

Also, a book that sums up good points about selling a web app (especially if you are a developer) is http://www.startupbook.net/

Good luck!