Carmack's company didn't have this problem when they made the games they eventually open-sourced. When they did get bought by a publisher, they no longer open sourced their games as far as I know, so who knows what the engineering practices were like at that point.
I don't know. I think it's silly for decades-old-games to run into licensing issues when trying to go open source. The tech is effectively obsolete except in the instance of that specific game.
I'm also not sure why you think remakes are that different from cover bands - neither are a 100% identical copy.
Those games are far simpler (by at least an order of magnitude, maybe two or three) than most of the modern AAA titles.
What was the last AAA game Carmack shipped? Rage? Hardly a smashing success, even if it was technically impressive.
I have a huge amount of respect for Carmack’s skills and work ethic and track record, but I wonder if he really has a good handle on the sheer amount of work that goes into a modern game. With often dozens of programmers, and hundreds or thousands of contributors, the coordination overhead is just massive. I think that’s why overall quality control is harder to maintain.