The motive behind criticism often determines its validity. Those who care criticize where necessary. Those who envy criticize the moment they think that they have found a weak spot.
Nostalgia is a necessary thing, I believe, and a way for all of us to find peace in that which we have accomplished, or even failed to accomplish. At the same time, if nostalgia precipitates actions to return to that fabled, rosy-painted time, particularly in one who believes his life to be a failure, then it is an empty thing, doomed to produce nothing but frustration and an even greater sense of failure.
This pain, this terrible seeing-through that is in me now. It wasn't necessary. It is all pain, and it buys nothing. Gives birth to nothing. All in vain. All wasted. The older the world becomes, the more obvious it is. The bomb and the tortures in Algeria and the starving babies in the Congo. It gets bigger and darker. More and more suffering for more and more. And more and more in vain.
A theory is like medicine (or government): often useless, sometimes necessary, always self-serving, and on occasion lethal. So it needs to be used with care, moderation and close adult supervision.