[ktizo]

what if you are building a startup to deliver ads?

Then you are starting in an overcrowded space. The only reason that the first internet empires have been ad-based is because it is the easiest and most obvious business model to try, given the existence of a large and rich existing advertising industry that is already used to supporting a lot of old media. It is not a particularly innovative business model however and is also probably not the biggest pile of money on the table.