So knowledgable for such a new joiner full..!
If you were privvy to the information, you would see the sicknesses were distributed fairly evenly between ranks. And yes, statistically, a larger proportion of those "unfit for duty" did come from DB, this would seem reasonable enough considering a larger proportion of the crew live in DB.
And as for the sackings... now I'm not smart enough to be a CX manager or anything like that, but I reckon, if I was short of crews, and then I sacked 50 odd, which did nothing to change the resolve of the remaining (except perhaps strengthen it) then sacking more probably won't be in my best interest.