The devaluation of 'the past' in so many of its forms, means that important lessons are forgotten and precious knowledge, which transcends time and place because they are related to universal human experience, is lost, and that people re-invent the wheel again and again. It is one of the destructive characteristics of what we best call 'conventional modernity', an overall mood of enthusiasm for what is not yet there, for total freedom unhindered by the usual human limitations and the necessities thrown-up by reality, in short: an immature mind set, permeating the whole of Western society, and which has turned into convention and 'received wisdom'. For arriving at a 'true modernity', i.e. one which is rooted in both reality and the awareness of the human condition, a modernity focussing on improvement instead of newness, it may be helpful to remember Cicero's saying that who refuses to know the past will forever remain a child.
Ironically, the usual, collective mindset of modernity is much like dementia: gradually the past, and thus any elements of identity, are forgotten, without something taking their place. It is a created dementia, entirely self-destructive. But it has long roots in that very past, as this interesting article in The New Statesman shows: