Isn't it just the cycle of tech captitalism? Build a product that pleases users, make it spread far an wide at a loss fueled by venture capital and hype, and then when you cornered the market, there is nothing to look forward to. No more insane year by year growth in traffic or users because you are limited by the number who realistically will sign up. You can no longer promise growth so you have to promise ever increasing profit, and then it comes to light your premise was never very sound to begin with. Youtube costs too much to keep running with the original idea, Reddit has lousy data gathering opportunities because they can't couple anonymous users to real people, There is no reason to even visit Twitter because you'll read any tweet worth mentioning by the thousands of sites who are built around taking tweets and writing articles about it, without all the overhead of actually supporting the Twitter infrastructure.

They are all decent ideas if you want a lot of traffic, not so much if you want to monetise it.

Microsoft doesn't belong in this list because they can honestly do what they want with their end user product, they have the office space cornered. The minimum requirements are so easy to bypass they might as well not exist. It is just a way of being able to cut down on the range of hardware they need to support. They have Azure and Microsoft 365. If they could ditch Windows completely while keeping companies glued to those two they would. Windows desktop sales are small change in comparisson.

The simplest description, and the one that stuck with me, is telephone extensions. You have one general phone number (your computer's IP), but to reach a certain program inside the computer you have to dial a certain extension (port number) as the program is only listening on that particular extension. Some of those extensions are predefined so you don't need the know them, they are programmed into the sender software (web traffic over 443 (https) or 80 (http) for example, others you need to know the extension to be able to communicate (a friend setting up a Minecraft server for example). A firewall can block certain types of traffic by not allowing traffic to certain extensions (you can't communicate with the torrent client if those extensions are blocked), port forwarding works by taking traffic for a certain extension and sending it out over another, ...