I think you raise a much bigger question than that addressed by the paper.
BTW, the abstract of that paper sets O2 contribution of phytoplankton at a much higher level than you do (70%).
But I don't think that will affect your argument.
The rate at which forests are being cleared and replaced with substitutes that produce less (or zero) O2, or produce greenhouse gases, might be a point of contention.
Also, the rate at which CO2 is being added to the biosphere, the capacity of the ocean to buffer that change, and when a limit might be reached.
I am guessing that Petrovskii will contend that if we are adding CO2 at a rate much higher than in the known history of a human-life-supporting biosphere (1MMYA), and if there is a limit to the oceans ability to buffer that change, and if we are actually clearing plant life faster than it is growing, then we might run into a problem with high CO2 and low O2 in the atmosphere.
But I woudl be interested to hear what he says.
I just posted this because I thought it was interesting that O2 is an issue at all. Also interesting (to me) is the fact that plankton dies off at higher temps will make it difficult to employ any climate engineering that depends on phytoplankton as well (like iron seeding).
In any case, after a naive look at human capacity to handle 7% oxygen in the atmosphere, I'm pretty sure our bodies would adapt, even if it happened over a short time period. The evidence is fairly clear on that one.