It's difficult to make any strong guarantees about hypothetical situations years in the future.
So long as people are still paying for C3, then it's perfectly economical to keep supporting it indefinitely.
If nobody is paying for C3 any more at some hypothetical point in the far off future, then presumably few people will care what happens to it, especially if there is a newer and actively supported product that can still import C3 projects.
It's also possible that some future Windows update makes C2 unusable long after its support ended - so contrary to what you imply, there is no guarantee that is going to last forever.