While the decline in O2 doesn't present any immediate danger, it does bring up an interesting scientific question: why has the Earth's atmosphere's O2 concentration remained within a certain range during the Phanerozoic era (since the Pre-Cambrian)?
Oxygen is produced by photosynthesis, but build up of O2 in the atmosphere is governed by the balance between net O2 production due to burial of organic matter (organic matter left at the surface gets consumed and re-oxidized, causing no net increase in atmospheric O2), and exposure/oxidation of reduced materials in eroded rocks and of reduced gases from volcanoes.
There is no a priori reason why these two processes have to be in balance, and both are expected to be quite variable, affected by climate, tectonics, changes in species, and amount of erosion. Absent photosynthesis, the time scale for the latter processes to deplete the atmosphere of oxygen is only 10 million years or so. Oxidizing reduced materials (organics, but also ferrous iron, other transition elements, and sulfides) in the top 200 meters (IIRC) of the Earth's continents would consume all the atmosphere's oxygen. So, why has O2 not gone to zero (or close to it) at some point in the last 600 million years?
One might argue there's some negative feedback effect, but none has been identified. You might think fire would do it. The idea would be that at high O2 levels, fires would burn even wet wood, turning the world into a blazing inferno with less photosynthesis (never mind that this would not affect phytoplankton, which causes much of the net burial of organic matter). But during the Carboniferous, atmospheric O2 levels reached nearly twice what they are now. That was when 90% of world coal deposits were produced. And fires might even make organic burial worse -- a positive feedback -- by producing charcoal that's highly resistant to decomposition. A significant fraction of coal in Carboniferous coal deposits is in fact charcoal, thought to be from wild fires sweeping across swampy forests.
The explanation for the stability may be anthropic selection. Had O2 crashed to near zero, higher life (including vertebrates) would have been wiped out, and we would not be here. If our descendants explore the galaxy and its Earth-like planets, they might find that this long run of luck is extremely uncommon. If this is the case, then humanity is going to have to take control of O2 levels in the future, as past luck is no guarantee of future luck.