No, current astronomical knowledge does not indicate that the year is exactly 365.25 days long. So no, birkat hachama does not represent any "objectively" significant event.
But then, neither does your birthday. The choice of one day a year for this celebration is surely not based on the fact that the earth returns to the approximate point in its orbit it was at when you were born. That fact is incidental, and the birthday's real significance for all but astronomy buffs is that it is a convenient milestone for stepping back to evaluate your life.
Birthdays, or Pesachs, or Yom Kippurs, are easy milestones to commemorate. It is easy to see where I stood last year, to evaluate what I have accomplished and how I have grown since then, and what I hope to accomplish in the year to come. A year is long enough for real accomplishment, yet short enough that the flow of human life does not make comparison between one year and the next impossible. Once a year would be an appropriate opportunity for reflection even if the seasons did not make that exact period inevitable.
Let's see what happens when I try to approach birkat hachama the same way. Where did I stand 28 years ago? I was not yet born. Where do I hope to be 28 years from now? Trying to picture myself at age 53, the first thing that comes to mind is that I hope not only to be married and have kids, but that those kids will be starting to marry and have kids of their own. In other words, my generation will be replaced by the next generation. Looking one 28-year period forward and back, I am right away confronted by my birth and my mortality. That is even though I hope to live much longer than 56 years.
This message may seem like a depressing alternative to the birthday. But I think that the messages complement each other. Birthdays remind me of what I am capable of accomplishing. Birkat hachama reminds me of the inevitable limits of that accomplishment. The world was created for me, yet I am dust and ashes. As we go about our lives, we should have both halves of that proverb in mind.