Not many Koreans today must be too proud of the economic and political developments in their country over the past 20 years. It has been a period where rapid economic development gave way to slower growth and increased foreign debts, resulting in the catastrophe of the late 1990s.

The underlined part of the sentence...

Is that well written sentence?

That is the same as "many Koreans today must not be too proud ~~~"

So, I feel "too" should be changed into "very"...

Also, I feel that there must be some better expresion without using "must".

And how about, "Not many Koreans today must be being very proud"?
I think this reads better:

* Not many Koreans today are likely to be very proud of....
are likely to be ....


That is beautiful expression!