Or...charter a Bizjet and use it 9/11 style??
or
Transmit false ATC instructions to cause problems.....
We could go on and on......
It doesn't take much imagination really, but we can't worry about every single possibility. We can't restrict our lives so much that we live in constant fear of attacks from here there and everywhere...
YES, someone could crash a light plane into another plane in all of the scenarios mentioned above... but is it a credible threat? NO!