I think you probably misinterpret my words. The author means a comparison between the old days (around 1970s) and present days. So I think it might be inappropriate to to revise to " Over the past three decades". Here's the URL link about the original text.
And I really didn't know there are two kinds of steamed buns in China before read your post because I don't eat it much.
I think corn buns probably are cheaper but buns made of flour are expensive. And in the old days, Chinese peasants could not afford to white steamed buns but steamed corn buns probably were available for them since it is cheaper economically.
On the base of that, it is not good to omit "white" from "steamed bun" though my translation is not accurate about it. I admit that I translated it literally without consideration.
Any other comment?