All you have to do is watch them make it. It's nothing like it was in the 80s/90s. Maybe it's better, maybe it's worse, but it's not the same.
It used to come in a drink cup. It used to come from one of three handles on the shake machine: Vannilla, Chocolate, or Strawberry. Now it is just vanilla and they throw in a syrup for Strawberry or Chocolate.
It now comes in a "parfait cup" with whipped cream and a cherry, like a sundae. I suppose you can ask for it in a drink cup without whipped cream and a cherry, but you know how special requests go at McDonalds... You'll either break their brain or they'll just tell you they cannot do it. That is beside the point, anyway, as the milkshake has changed dramatically.
The company is hurting very badly. They are on life support regardless of stock indicators. Just open a Wall Street Journal. The all-day breakfast signals the beginning of the end. It'll spark a bit of an upward trend for a bit, but by year's end they'll need another boost. We'll see.
Maybe the answer is a "Heritage Menu" of the original basics made in the old school manner.