What do you call it when the current generation thinks the previous is better/more poetic?

It's referred to as the "allure of nostalgia" in the wiki entry for Midnight in Paris.

It is also referred to as the Golden Age fallacy in Midnight in Paris.

It's a mixture of romanticisation of the past, and minor negationism; ignoring the negatives and focusing only on the positives.

It's closely related to "the grass is always greener".

There is a commonplace longing for the good old days.

For example, there is often a longing for the turn-of-the-century era (1900 not 2000) that is thought to be kinder and gentler than the later 20th century, as exemplified in Hollywood sagas like Meet me in St. Louis. A reading of some authors, like Stephen Crane's Maggie, Girl of the Streets, paints a much more realistic and somewhat brutal picture of that period.

As Ms. Simon points out, anticipation of what is to come may be ill-advised, since these are the good old days.