A rich individual may buy one lemmy instance, can they buy them all?
Yes, easily. Additionally, they can control the search engines which lead there or not, the commonly used platforms for code sharing so that development is more difficult, and so forth. They have an incredible advantage through economy of scale. The barrier to entry for putting any information on the internet may not be very high, but making it findable and accessible has a very high barrier, and relies on corporate-controlled platforms.