I guess I don't understand what you mean for Mathcad on one side on not on the other side.
I understand that Prime is not what or where is should be. I have spoken with the PM's on this quite frequently as we cannot go to Prime yet either. But I also don't expect it be opened by older versions. There are many reasons for this
1. Newer or different features
2. code changes
3. the engine that drives it all is also different.
Mathcad Prime was a complete rewrite of the code. There should be no expectation for it to be backwards compatible.
If I create a part in Creo Parametric 3.0 and if something doesnt work or another user on Creo Parametric 2.0 wants to use it, I dont have any expectations they should be compatible. They are not the same software.