That was not the argument the OP made. The argument wasn't that other people aren't necessary for income. Their argument was that relying on one, and only one 3rd party platform for selling things online was a bad idea.
If your career and income are entirely reliant on one company, regardless of type (tech platform or retail or college or whatever), you're beholden to their whims and whether or not they stay open. It's a good idea to diversify your income if possible (via savings and investment if possible).
Further, this is even more of an issue if your income is selling and buying via online platform. They may not shut down, but they very-well may change the TOS to exclude your type of selling. . . . Which is what this is.
