About how synonymous will the two decades be to people in the 2020s, 2030s, etc?

I'd say about as much as the 1960s and 1970s are to people in the 1990s and 2000s.

Why?

Let's face it: the only things that really pop-culturally divided the 70's from the late 60's is disco, funk, and the whole Afro thing.  As far as I know nobody bashed the 60s a lot in the 70s and many things from the 60s, such as the whole "color" thing and words like "groovy", lasted into the 70s.  The two, while seen as different for political, technological, and certain cultural things, are interchangeable in what they mean.

About the same could be said for the 1990s and 2000s.  Most of the observed differences between the two decades revolve around the great different (but quite continous) political trends and economics.  If you look up 1990s on Google many of your results will talk about the Bubble era and such.  The pop cultural differences between the two decades that are noted, such as Grunge, are quite small and are not 100% unique to either and tend to have some in both. 

That's not to say that the 60's and 70's are one and the same or the 90's and 00's, but I do think that in many ways they are two parts of one whole.

Here we go again...  ::)

Yeah we've discussed this topic before, but I agree with him.  I have always thought that the 60s and the 70s sort of ran in with each other.  Although they had their differences, they were very much the same animal.

The 90s and 00s are the same way.  There are plenty of differences, but it's basically still the same pop culture animal.  We will probably have to wait for the 2010's for something different.