You make a good point. I won’t argue trademark policy with you, but there is no reason to believe that removing the Big Shef name from Hardee’s ownership will resurrect the old burger.
McDonalds, Burger King, Wendy’s, KFC and virtually every other trademarked brand has changed over time. Coke today is not the coke of years ago (and don’t forget new coke which Coca-Cola was perfectly within its rights to pass off as the real thing!).
The best chance any Big Shef lover has of getting their favorite old burger is probably to revive the Burger Chef system, open using different names, and hoping there is enough market interest to support the concept. I’m not banking on it.