McDonald's cuisine definitely has not improved in any way over the years and I agree it is a whole lot worse in several areas. There egg mcmuffin once was a reasonably (for fast food) nutritious snack when you had to have breakfast on the run. Now it is a mostly tasteless and greasy mess. And I had a big mac more recently than you and never ordered another for the very reason you mention. Or maybe it's just that our food preferences have changed over time? I really don't believe it's that.
The only thing McDonalds does really well now are its fries which are exceptional, but I rarely ever eat fries any more anywhere.
Alas. All 'progress' is not good.