This is besides that in the closed cramped quarters of the plane it may be more likely to spread.
Exactly
Again, you are mistaking statistical overall risk with incidental risk. Obviously, I can't know what your risk on one particular flight is but we can estimate the risk of taking one flight among thousands in a world of 7 billion people that even if they are undereported has maybe a couple hundred thousand cases. I'm trying to focus on the thought processes that cause hysteria. I also don't know when or where the next lightning strike will be but I do know my overall risk of getting struck is pretty low. But hey lots of people waste millions on lotto tickets too so I know people overestimate their odds for the both the positive and negative! 
TLDR, I don't think you understand risk. Bottom line COVID-19 is a pandemic, traveling during a pandemic is risky and it will get increasingly risky until it peaks. There are literally cases in almost every country, and you don't need an entire infected flight to get infected. It can be a single passenger or more likely, a single FA that becomes a super spreader.