Just to add another data point, we're small indie (games) developer, we spent over a year on our first title with very little marketing budget and these are our sales figures from our launch: http://www.gamasutra.com/blogs/YacineSalmi/20160519/273030/E...
In short, decent sales but not enough to recoup our costs. Still, we never expected that we would hit gold on the first try. I think to be as sustainable on iOS you either have a successful niche product or a collection of product, with each release building a further revenue stream.