The likelihood of catching it while next to someone who has it isn't small but the likelihood of catching it on any single flight is most definitely TEENY!
You are saying the risk on any single flight is TEENY as if that is a fact, but the risk may in fact be huge. We can't assess risk until we know how widespread the virus is and it takes time to find out. Are you traveling to a place that implemented extensive testing before a breakout, and has the proper protocols in place? Well, there isn't a single country that started testing potential cases before their breakouts. Italy, South Korea, Iran, and other countries are starting to find out now how many people were infected the last couple weeks. You don't think that any flight in those countries was pretty risky a week before they started confirming cases en masse?
Who is the next Italy - do you know? It can be Germany, Spain, Brazil, Canada, France, and heck maybe even the US.