Is there some reason to get all the gold in the retro levels?
Solution 1:
There sure is. You see this "Totales Grande" on the world select screen?
That gold counter tracks each level. It doesn't count the different difficulties separately, so once you get the gold from that level, you're golden. (No pun intended)
The kicker? The total is out of 5,000, and after beating all of the levels, you might find yourself a few hundred shy of the even 5k. Retro Warp Zones count towards this counter. If you want to get the full 5k, you'll need to ace all the warp zones.