There are two scenarios in which halving the number of droplets inhaled has little effect on the risk of being infected, and two where it is moot. The ones whe

There are two scenarios in which halving the number of droplets inhaled has little effect on the risk of being infected, and two where it is moot. The ones where it is moot is those where either you you are already infected or no-one is. Halving the number of droplets will have no effect if droplet inhalation is not causal in transmission. In this case, there is plenty of evidence that it is, in fact, the primary mode of transmission. It would also be the case when, even at the lower droplet count, one is likely to become infected. This is essentially the scenario in your second point. None of the above are the case in a typical bus ride today, and for such, it is reasonable to suppose, as a first approximation absent further evidence, that risk is correlated with the degree to which one is rebreathing other people's exhalations, even though one can postulate that it is also correlated with other factors. One can question the source of the CO2 on a bus, but the measured values reported here are significantly above those measured by busy urban roads, where 1000ppm is considered vary high. If the bus is leaking CO2 into the cabin to any significant degree, there are more immediate health concerns. Demanding near-perfect data before considering its implications can be a way to avoid issues.