Well, the point is that the advantages of centralized development don’t have to be given up, because development can still be centralized. The advantages of FOSS development I’m building this point upon aren’t like increased efficiency or something like that. It’s an ethical thing, allowing the game to be the public good it ought to be (and functionally kind of is, looking at proxies and homebrew). If those original designers ruin the game in a way that upsets enough people, a new designer or group could fork it and become the new standard. This isn’t really possible with a proprietary game without stepping incredibly carefully around the law. Homebrew and modified cards can exist, but if there was a modified version of Magic threatening to replace the original game, Wizards would be sending nukes your way real quick.
But I get that you seem to be coming at this from a different position, if you don’t consider games being made as part of the commons as an inherently good thing then we have a philosophical disagreement that goes beyond the scope of this discussion. I believe that making stuff that belongs to everyone IS the value of free and open source development, not a means to an end.