I think this is simple advice and often incorrect.

For example: JIRA, it's very complicated software. Many of the complicated features I think are essential for it to get into the Enterprise market. Revenues $242 million.

Basecamp: Their mantra is simplicity (although they do not actually do just one thing, they have several features as part of their software). Revenues estimated at over $100 million.

I do no think there is one simple answer to how to build a successful startup. Focusing on being great at one thing might be a good idea for some businesses but it's not only way to do things and it's far from proven as "the" way to build a startup.

http://www.afr.com/p/technology/atlassian_sales_leap_as_reve... https://medium.com/@hungrycharles/basecamp-the-small-bootstr...