I wonder if companies are experimenting with materials like UHMWPE for non-elastic, high strength-to-diameter tendons.
I dont know if you'd have to weld the dyneema to the anchor points, though
A lot of problems in robotics reduce down to continual learning. Essentially all system identification tasks become obsolete the moment you have a self learning system and yet we have an AI industry preaching that AGI is around the corner without this "crutch".
Another idea is to use an external camera or two and to track the fingers with a deep learning model. But this can become messy if other objects are in view. And it might also introduce more control delay in the feedback loop than a simple sensor.