It's just impossible to make any predictions. It could be a horrible couple of years with orders being cancelled and airlines going bankrupt, or the industry could contract this year and then start recovering again next year and boom for a few more years.
It's a cycle, at some point it will go up again. How much will it sink? How much will it bounce back? Impossible to tell. I think the next 2-3 months will be crucial to get a picture of what the industry will do. If restrictions start being lifted gradually, we may experience a gradual, slow but steady recovery, with pre-Covid levels to be reached in a couple of years.
Much depends on the outcome of a possible vaccine.