The USD itself has existed for over 150 years. While I do understand your point it's markets at a whole that decide these things. The USD is very good at storing its value if you were to compare all the other currencies from the plethora of countries out there. Keep in mind there were other currencies also pegged to Gold & Silver.
In the late 1890s there was a dem politician, William Jennings Bryan who played a part in this. I guess its one of those things that caught on.
Edit: Didn't ready that you mentioned 3 decades is too, long. Actually it's too short, a currency's best test is how it stands inflation & recession - just they come around once a business cycle, which is excruciatingly long.