They could just permanently 'toggle disabled' the code or something, so you could at least review it and replace it below those lines
As I understand, the reason why it's not possible in C3 is because its code editor is visual. It can not display an event or action from the missing plugin, because it doesn't know how. (this info is in the missing plugin)
The only option is to skip all these events, actions and conditions. And there is a good chance that the project will become corrupted without them - there may be orphaned Else-blocks, missing functions, broken hierarchies etc.
Also, I guess, Scirra developers want to avoid complaints from people whose projects became unusable because of that.