I see your point, and it's true that a lot of earlier propositions of socialism are already implemented and seen as desirable by most people, but I think that's something to be said about left wing in general. We're no longer fighting to abolish 20 hours work day, instead it's 32 hours work week now. As the window has moved, so did the policy. I've never heard about there being a stopping point in regards to how far left socialist democracy system is supposed to push before it's "principles" are satisfied, but my understanding was that it's at least until capitalism is abolished (and socialism emerges) or capital class is weak enough to be defeated via revolution of some sort. And so, since it's intended purpose is to push much further left instead of mantaining the current system and it's status quo, and some changes required to implement it's propositions are radical, I feel like it can't be reasonably called centrist system. Unless your definition of political center differs from mine, but it would require it not to account for both how far left the system intends to shift the society eventually, and also how radical the changes are in comparison to the current center. That, or what we understand as socialist democracy differs, wikipedia page for it mostly fits my definition, and I don't know where to look for more 'common' view on it.