[BullMcGiveny]

Well the reason they added that retcon was that a storyline was written about Natasha being a kid in WWII, meeting both Cap and Wolverine, and people wanted to keep it in continuity.

The cold war is a big part of Natasha, and those decades of experience lend credibility to her rep as the world's greatest spy.

While I would agree on Steve being the only super-soldier, I would point out that Natasha's serum makes her ageless. So it's more Infinity Formula than it is Super-Solider Serum.