I'm not trying to argue that pneumonia isn't a potentially serious ailment. But from what we have seen from Hillary this weekend:
Is she dying? Likely not.
Is this disqualifying? No.
Is this a serious case of pneumonia? Most signs point to no.
Are there signs of a more serious ailment her doctor isn't disclosing? No. This is all par for the course for pneumonia.
Pneumonia takes time to get to its worst. It isn't fine one moment and than BAM! pneumonia and dying.
I had it when I was at University. I had been feeling shitty the latter half of the week and on the Friday went to lectures after taking every cold medication under the sun. Then the weekend came and I decided to travel home to my parents' as I thought I might have the flu coming on. On either Tuesday or Wednesday I called my Dad at work and (kind of) said "I can't breathe and I am coughing up blood". I was in hospital the same day and stayed there for a week.
So I was serious enough to be hospitalised for a week, yet when it was first coming on I was still going to my lectures and, IIRC, going out in the evenings.
ETA:
The point is, we don't know where on that kind of timeline she is and speculation will get us nowhere. We can't decide if it is serious or not as Pneumonia is a spectrum. But blanket statements saying she did this and that and thus it is not serious is ridiculous.