That part at the end of your comment, "...that no one but a single old developer knows how to solve." resonates with me on one of my favorite game franchises from the 80s. Alternate Reality. It was planned to be a 7 or 9 game franchise, all interlocking with The City but only The City and The Dungeon ever got released.
From my research to find the rights holder to that series, which came up a dead end, I did learn from talking with people that worked on the ports to various computers that the original developer kept a lot of details behind the scenes (there are over 10 stats that are not even visible to the player for instance).
To port the game one would need to know the details that only the original developer knew and as far as I know, they are long forgotten at this point as the games were programmed during a period of strong use of post it notes around the monitor. Details that are simply no longer available.
There are many classic games, especially on computers, that are like this which complicates porting even more once an owner is found.
I've heard stories like those sticky notes. People had a somewhat similar problem with Half-Life, with a single mechanic, but they managed to eliminate it from the equation and glue two pieces of code together. From what I've heard, this was a fairly common practice. Especially since programmers like Gabe Nevell and John Carmack often sat after work hours and tested every solution they could think of. Did something work? Did the tests confirm it? Then we write the solution down on a piece of paper... Or maybe not, and then think in that jumble of code about what the author meant :D.
Oh yes, porting games from archaic platforms is a problem. Besides, some PS1 games were difficult to port due to their technological limitations. I don't remember the details, but some/many PS1 games had some graphical limitations.