In some respects Civ II is the best of the franchise. Love III and IV, and AC, but 5 is also very good, but only if you play the Shoshone. ;p

Quite the opposite. Whatever technical problems there are, can be solved by software engineering.

Copyrights expire in many countries, like most European countries, 75 years after the death of the last living author. So even if the last credited author dies today, that only starts the clock for the next 75 years.
Obviously there are exceptions to this, and whoever owns the copyright can announce it freeware or public domain or something.

But the bottom line is, you won't be seeing any commercial games, or even hobbyist games for that matter, to have their copyrights expire during your lifetime, unless you are planning to live for a very long time.

If there was any 25 years rule, then Good Old Games really wouldn't have ever existed, because it would have been simply easier to wait a few more years to get those 80's and 90's game as public domain.

My favorite Civilization games were spin-offs: Call to Power 1 and 2. I liked the vast time periods that they covered. We started in ancient times and lead the civilization to a very distant future (cities in space, starships, lasers etc.). These two were the biggest Civilization games when it comes to covered time span. Call to Power 2 is available on GOG and I recommend it to every fan of turn-based strategies.