Double cheeseburger has always had and still has two slices of cheese. The McDouble (which used to be $1) always had a single slice of cheese.

The real hack was asking them to put Big Mac sauce on the McDouble. For $.30 it was pretty damn close at 1/3 the price.

In the last 12-24 months the price of fast food in particular has risen at a higher rate than inflation has hit other types of food and goods. Fast food makes no economic sense anymore.

And you're right, the food has gotten worse as well.

The Big Mac Index has the fatal flaw in that it assumes the value of Big Mac is consistent over time; McDonald’s has been at the forefront of fast food attempting to break into a more high income market segment.

Yeah, same here. Perhaps the Big Mac Index [0] is what you want.

[0] https://en.wikipedia.org/wiki/Big_Mac_Index

Of course this is comparing to one data point that is an outlier. If people are choosing to pay $3 in today's dollars for it, that means that in 2010 McDonald's was underpricing that item relative to its market value. Presumably deliberately as a promotion. Compare across everything you buy and compare like-to-like if you want to judge its accuracy.