The hypothetical situation he suggests is that Twitter is cut off from its customers. That means zero income, and customers making new lives on other platforms. Musk has huge debts, and huge expenses.
Let's say it takes 10 years (the opposite of quick). That would leave Musk spending $50 to $100 billion of his own money to keep things going. At the end of that, he would be left trying to remind people of that app they used 10 years ago, but have not thought of since. In theory, people would have found better apps by then, or at least apps that their social network uses. There would be no way to make it work.
Then you are a fool. Making an entire smart phone, from scratch, avoiding patents and copyrights, is very difficult. It is what we used to consider a supercomputer that fits in a pocket. The software alone is like writing a million books with no spelling, grammar, or plot mistakes.
