"That means that it's good to be big, because you can spread those fixed costs over many, many users. One programmer writing software used by five hundred million users can make a lot more money than software used by five users. So the typical model is to take in a lot of investor money, operate at a loss, and lose money while offering a very compelling service to grow the userbase as quickly as possible.
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.