The idea that the risk profile to manned AC goes from zero to infinite bajillion upon crossing the 400' AGL altitude is unhinged.
The risks are certainly much greater at 3000' AGL. Near non-existent at 400'. It doesn't get riskier in some sort of linear ramp between the two.
Flying at 800' AGL is, in reality, not much riskier of colliding with a manned aircraft than at 400'. Sure, there is a slightly greater risk, but then this can be somewhat mitigated by care and attention by the sUAV pilot. Regardless, there is greater risk -- how much, though, is the fertile ground for debate.
I have experience as a private pilot (small 2 and 4 seaters, VFR, got my ticket in the 80s), as an RC hobbiest (gliders, helis), and now an sUAV pilot. FWIW, I don't think the risk of collision between 400' and 1000' is much greater than below 400', as very few manned aircraft fly that low anywhere except around airfields, and flat open plains.
Again, this is not about whether manned aircraft CAN, or ever DO. It's about the practical reality of how often they ACTUALLY DO, and therefore what sort of risk there really is in going a bit higher than 400' with your Mavic. One of the reasons pilots stay above 1000-2000' AGL for most flying is, ironically, to avoid birds.