Archaeology is the study of the past, using physical remains. One of the fascinations of this study, to me, is the way that archaeology reveals the actions of our predecessors in the landscape - and what we interpret those actions to mean in terms of their society and attitudes.
Societies throughout the ages have remodelled the landscape, and we have been able to see what elements they respected and retained (or built around, at least), and what elements they simply ploughed through (metaphorically and actually).
The changes we are making to our landscape will be the subject of study for future generations of archaeologists.
Whilst I am fundamentally in favour of the preservation of aspects of our past, I cannot help but feel that this is a very selective process - we, as a society, seem to prefer the preservation of "nice" things, whilst "eyesores" are waved a thankful goodbye. I would like to see a lot more preservation of ugly stuff as well as nice stuff - but not necessarily always by preservation in situ.
If buildings such as these mills are to be torn down - and frankly at some point they will disappear - then should we not take great care to preserve them in the record? Can we not involve archeological techniques to record the standing buildings, and follow through during the demolition to record aspects currently invisible?
This would provide our successors with the sort of information that we would be delighted to find when doing desk based assessments of sites today.
I would also suggest that this sort of work should be done for "ordinary" buildings as well - when we demolish a 1960s office block we need to ensure that there is preservation by record to go with the traces we will inevitably leave in the ground.