That's about 5000 feet in ten million years for the sandstone. Pretty much takes care of that, shouldn't be any left standing without even multiplying the "tens" of millions.
If it erodes 5 feet in ten thousand years and it's five feet in radius, we'd expect it to be gone in ten thousand years. We can extrapolate backwards to estimate how long it's been eroding. I don't know why you think you can tell that there "shouldn't" be any left. You don't know when it started eroding.