This very simply isn't true, or at least it's not the main driver behind the riders' actions.
1. The majority of riders is not sick
2. There were far less riders sick the last time a Giro stage was shortened with the excuse of cold, wet weather
3. The weather isn't that bad today, so there was never going to be a bunch of racing in bad weather that would have aggravated any illnesses today
This is a valid argument against the fact that the stage has been shortened.
However, if you actually listened to the riders, they have all explicitly mentioned the weather from the first two weeks as a key reason why the stage has been shortened. The crashed on Tuesday/Wednesday, the illnesses, the recent-record dropouts, will all play a part. If this was the first stage with bad weather, I doubt anything would have changed. I might be wrong here, it's a counter-factual after all, but it's pretty self-evident (and indeed evidenced by the cyclists in the race, from Thomas to Hansen) that the first two weeks have played a
major part in the stage being shortened. Which has a lot of very important issues connected to it, but without addressing it you'll just be discussing imaginary issues.