So long as these companies maintain their dominant positions, and no major market shifts happen (such as introduction of the global computer network), it's not

So long as these companies maintain their dominant positions, and no major market shifts happen (such as introduction of the global computer network), it's not likely that someone can challenge them directly. However, they are being challenged by groups of smaller startups. For example, Facebook is losing its appeal due to apps like Snapchat, Houseparty, etc. Amazon is being nibbled away by direct-to-consumer companies, like Casper and Dollar Shave Club. Google is getting challenged by voice assistants, chatbots, and so on. A good way to think of it is not as who can challenge these companies, but which human needs can be served better.