IMO the phrase "legitimate investment" should be reserved for situations where you spend money something (e.g. kitchen equipment) that allows you to create new real-world value (e.g. food) which you can hopefully sell for a profit (it's still a legitimate investment if that fails). It should not be used for Ponzi schemes, gambling, outright fraud, or anything of the sort. Buying something and then hoping its price goes up before you sell it should not be called investing, but gambling - unless it fits in the category I just described.
People find value in acquiring things they want. For example if someone wants to have a one letter username on X, there is value for there to be willing to sell one.
