One may learn the important lesson from a recessional time: the consideration of why the major Romantics chose to incorporate, both into their literary endeavors and their generalized consciousness, energies from without the charmed circle of the human race, rather than within; to forge a workable relationship, worth writing about, with trees, mountains, rivers, birds, flowers, and the like. One answer is painful, but simple: the consciousness of a Keats or a Shelley has more in common, both in its intentions and in its creative capacities, with what inheres in natural objects (trees, mountains, etc) than with the average human being, and with average human consciousness. It's a byproduct of both age, and experience; to understand, on a profound level, how middling most human consciousness is, how involved in delusion and duplicity, and then to see how this charmed, or not very charmed, circle might be broken. If you can access higher realities in a meaningful way, there would seem to be no reason not to do so. In terms of a lesson from Romanticism worth learning, that is one, though it may or may not be the most salient.
What Modernism and post-modernism gave us, where literature is concerned, is the sense that these relationships, between the human mind and the Otherness of nature, are silly, adolescent, frivolous. The problem is that most human consciousness is, in and of itself, silly, adolescent, and frivolous, and to stay within the charmed circle of the human (or, to get even more narrow, the charmed circle of textuality) is to stay a child, repeating ad infinitum that we are the center of the universe, and that the human race should be homogenized the right way. A homogenized human race manifests no individuals, and if there is nothing outside the text, the cosmic egg is both cracked and unusable. Why Keats and Shelley are older than those who followed and inverted them is that they bring to the surface how wildly uneven both the human race, and human consciousness, are, and that higher consciousness, when it manifests, needs to recognize both this variability (rather than a vaunted homogeneity) and the means to transcend it, sometimes within the charmed circle of the human, sometimes not. The flimsy quality of Modernism and post-modernism takes what makes Romantic poetry superior and pretends it is the pursuit of unreal phantoms; it's just that the human race, more than nature without us, has a problem with the unreal and with phantom systems of government, and when consciousness cannot attach to higher realities, it falls into a trough of stale ironies, incomprehensible symbols, and perverse lecherous inversions of lowliness into sublimity and cacophony into harmony.