So far, due to luck, testing constraints, and highly infectious nature of COVID-19, airlines have avoided blame as an environment where COVID-19 has been transmitted (they are of course responsible for the international spread of the disease).

BUT, with contact tracing and testing in place in some countries, as well as air travel starting again it is only a matter of time before super spreader events are identified that occurred on airplanes.

By way of comparison, we know SARS was spread on airplanes:

https://www.nejm.org/doi/pdf/10.1056/NEJMoa031349

Lastly, there are health exceptions to wearing a mouth and nose covering if you cannot breath easily with one. If you are healthy and cannot breath easily then I suspect you are wearing the wrong type of mask. I predict that wearing a mask will likely become a pre-requisite for flying (hopefully only until the pandemic is over) in the same way as wearing a seat-belt is (i.e. the announcement would be "when not eating or drinking, you are required to cover your mouth and nose"). Of course some people won't follow this (as today with seat-belts), some people will complain, but the vast majority of sensible reasonable people will follow the instructions and everyone will be safer for it.