Lol, what are you talking about? Metroid Prime is retro? Is Halo retro to you as well? How old are you? 13? No game that is 15 or even 20 years old is "retro". 

They're moot points though because Returns is a different era for a start you're comparing the 2D era to the 3D era where 2D's upfront selling power is vastly reduced as the industry operates on a different dynamic to back this is why the argument is flawed. 2D games when Returns was released are mostly filler titles in today's industry no matter how good they are only certain fighting games tend to buck this trend in SF and Smash, the original DKC trilogy were ground breaking because they were 2D games built for the 2D era while Returns and TF are 2D platformer games long into the 3D era they were never going to generate the same buzz no matter how they turned out TF is even considered by many to be better than any other 2D platformer but still had a subdued reception on reveal because everyone wanted Prime a 3D game as many don't consider 2D games ambitious no matter how much work goes into them.

To bring up sales in a selected period as a way to push your point as result isn't really putting forward a balanced argument because one is releasing in a year where the are games like Red Dead Redemption, COD, AC, FF, KH as well as the blue ocean titles (Wii Fit plus etc...) the other is only contending with games that are on the same playing field and this is highlighted in your own point when you say the third game in the original Trilogy didn't do as well because of the next gen platforms as that's when the 3D jump occured and also I disagree it wasn't one of the most exciting games of the year in the way you're trying to put it was heavily overshadowed in 96, Mario 64, FFVII, Resident Evil, Crash Bandicoot, Pilot Wings, Duke Nukem 3d etc... Even on the non 3d front the most exciting game that year was Pokemon. Another factor you don't take into account the were only two major competitive hardware platforms back with DKC compared to 3 plus PC now.

This is why total sales is more reflective of reception as Returns still became the second biggest seller in the franchise.