That is likely true for a majority of "the good stuff", but making that determination can be tricky. Let's consider spam emails. In our daily lives they are useless, unwanted trash. However, it's hard to know what a future historian might be able to glean from a complete record of all spam in the world over the span of a decade. They could analyze it for social trends, countries of origin, correlation with major global events, the creation and destruction of world governments. Sometimes the garbage of the day becomes a gold mine of source material that new conclusions can be drawn from many decades down the road.

I'm not proposing that we should preserve all that junk, it's junk, without a doubt. But asking a person today what's going to be valuable to society tomorrow is not always possible.