Once you're big enough, you can spread your costs around many users, so it's easier to make money. You switch from growing your userbase to making money from it. Because you aren't trying as hard as possible to draw in new users, the service is probably gonna get worse from a user standpoint."
This kind of reminds me of how Legos are made. Creating the plastic molds from a molding machine to make a single Lego is extremely expensive, but if you make millions of Legos in mass production it reduces costs to make them dramatically to a point where the Lego Group has basically no operating costs to make them anymore. That turns Legos into an investor's dream.