MLS is a lot of weird things, but a Ponzi scheme is not one of them. There is no "owner" of MLS that is profiting off of all this and paying early adopters off with later adopters money. The league is owned by the team owners. A ponzi scheme by definition has to fail, otherwise its just a business that succeeded using investment capital. If the league fails, all the owners lose their investments.
The gamble is that soccer will catch on in America and be the biggest league in the world because we have the most people and the most money. That's why the big players want in.