You asked to do the math.... Here I go....
TBC: Classic + TBC = Total amount of Dungeons (5 man and raids)
~
WoD: Classic + TBC + WotLK + Cata + Mists + WoD = Total amount of Dungeons (5 man and raids)
Any further questions?
The amount of content available constantly increases, and grows to a huge pile of content total.
It's a personal problem if you declare former content as magically non-existent as soon as a new expansion gets released.
But there are heaps of players who don't discard older content at all, and they have an advantage over you right there.
The game is what it is with all it's components available, not just those fractions of new content.