My apologies in advance if this seems a bit jumbled - not quite sure just how to phrase the question or the implications of it.
From my understanding, modern archaeology gains significant insights thanks to the behavior of previous generations and their propensity to just clear the surface of an area and just build on top of what was already there (exceptions of course being refuse sites, midden heaps, etc.) So digging within a classic city, like Rome or London, can turn up surprising finds of old houses, temples and the like.
Modern building styles however will generally clear an area fully, right down to the foundations. When we rebuild now, there is nothing that was previously there to leave behind. Granted, today's modern society is much more documented, and getting more so daily - so the future study of our times will be slightly easier. But what about 100 years from now?
For example, just a little over 140 years ago there was the Great Chicago Fire, and artifacts from that era still show up now and then following a (re)building project. But what about 140 years from now, someone building in a city where there was a massive tragic event that razed a portion of the city, what would they find?
Granted, we will still have our trash heaps to go through in the future - though, with even that technology improving, the degradation/disintegration rate is probably climbing - but what will archaeologists look for when wanting to see how the average person lived day to day?
I think it'll be a lot more about written sources, photographs, videos and such than physical archeology. We don't value architecture from more recent construction than we do older - I live in Toronto and I've seen pictures of the old beautiful buildings that once stood where my huge office tower now stands, but it was seen as "too new" to preserve. I think as long as we are able to store data such as newspapers, photos, and written work, the future will know all it needs to know, but archaeology will be more spent in a library with a huge database system than out in the field (at least when studying the present times). I think we also have more of an idea of preserving history, so we have saved some important items. For example, there are already rotary phones and WWII military uniforms in museums, so we won't need to dig them up.