You should do some reading on the altimeters in question. They were operating well outside their defined limits and parameters. If they had conformed to the specifications under which they were legally certified there would never have been a problem.

How do you harden the existing altimeters out there, equipped on thousands of aircraft?

What is the correct course of action when you discover thousands of pieces of vital equipment are badly defective? An immediate recall for all defective altimeters and, hopefully, fines for the vendors.