Midwinter is, at first blush, a stupid time for the new year, yes. But so is autumn; the changes in the natural world that you talk about are all deaths and endings. Even the harvest is simply the end of the growing season. Spring, however, is all about beginnings as new animals are born, and new plants stick their heads above the snow.

Spring is, in every sense, the birth of the new year and autumn is the death of the old year. If you insist on one year beginning as the last ends, rather than having a dead period through the winter, then midwinter is the only possible choice.