> However, the only way that Element can retain the right to provide alternative licenses to proprietary forks is if it has the right to do so, which means requiring a CLA

You wouldn't need a CLA if you didn't want to reserve the exclusive right to move from an open source model to an open core model. You wouldn't need a CLA if you didn't want the privilege to switch away from an open source licence at will.

Yes, a CLA is the only solution to retaining the right to un-open-source the project. The problem is that you'd want to retain this right in the first place.