So the risk you are looking to mitigate here is us going out of business and totally stopping support of our products?
If we did go completely out of business, shut the doors, and we all went and got other jobs and completely stopped selling Construct, Scirra is completely dissolved, at that point I don't see why we wouldn't give it away or open source it. I mean this event would currently appear to be highly unlikely.
As others have pointed out, even though Construct 2 is buy once, if we go under in the future and a new bug is introduced by Windows, Chrome or any other supporting software what then? This is a risk with ALL software.