Such low orbits should prevent that being a long term problem. I also wonder how much more difficult it is to target satellites going this fast than say the one China destroyed a few years ago?
Wouldn't a military power be more willing to trigger Kessler syndrome for 5 years than 500?
Note also that "Kessler Syndrome" is not a binary condition. As debris gets worse, the expected lifespan of satellites in those orbits drops, and as does the success rate of launching rockets through the effected altitudes. However these don't drop off to zero immediately. If the satellite is important enough to justify the elevated risk and additional expense then you can simply plan to launch several of them to get one of them through. Manned missions would be a bad idea, but odds are good that vital weather satellites and the like would be kept aloft.