I have heard this advice from many people: If you want gains, you need to invest in companies with room for growth, not in companies, who already have the maximum market share.
Let's take Google as an example. Let's assume they have 100% market share of online advertising. Let's also assume in 2018 the last person on earth goes online and there is no more growth for Google.
Why would the stock price stop growing, even if Google is in a great market position (monopoly) and very profitable and keeps increasing its pile of money? Shouldn't the fact, that it keeps its great position be enough for the stock price to keep rising?