I think there should be a sliding scale of how much one "counts on it" relative to planned retirement age. While I do think in large part it will still exist for decades, things like pushing up the FRA, or slight haircuts, aren't impossible to imagine.
Should any changes like that occur, I would be very surprised if they didn't get grandfathered by age. IE, everyone already on it or 5 years away? no change. 10? eh.... 20 - ok the goal post is over here now. that sort of thing.
In short, the earlier retirement one plans for, the less they should count on SS.